OPTICAL RECORDING AND/OR PLAYBACK SYSTEM

Abstract: An optical system (10) is provided for reading and/or recording arcuate tracks (16b) of digital information from or on a record medium (16). The system includes a rotating scanner drum (132) and a light source (38). The light source is spaced from the axis of rotation of the drum and generates an optical beam (18) having an intensity depending on whether data retrieval or recordal is desired. A lens assembly (34) including a focus and tracking lens and an objective lens is mounted on the drum and is positioned along the path of the optical beam between the light source and the record medium. Focus and tracking servo-units and actuators are disposed adjacent the lens assembly. The servo-units and actuators function to adjust the position of the focus and tracking lens axially and transversely with respect to the optical beam (18) so that the optical beam impinges on the record medium properly. An eccentricity correction function (23) is provided which adjusts the position of the record medium in the system when a track of information is to be read therefrom. The function adjusts the position of the record medium in a manner so that the arc traversed by the lens assembly (36) over the record medium aligns with the arcuate track of information to be read. This is achieved by moving the record medium so that the centre of curvature of the individual tracks recorded on the medium intersect the axis of rotation of the scanner drum.

Description: OPTICAL RECORDING AND/OR PLAYBACK SYSTEM TECHNICAL FIELD The present invention relates to an optical system, and in particular to an optical recording and/or playback system including eccentricity correction. BACKGROUND ART Optical recording and playback systems for reading and/or recording information from discontinuous arcuate tracks on record media are known in the art. In these systems, since the optical beam leaves the media and isolates the focus and tracking error sensors from any feedback signals, the systems typically suffer from problems in proper focus and tracking. This is due to the fact that the focus and tracking servo-units when isolated from focus and tracking error signals, tend to allow the focus and tracking lens assembly(ies) to wander. Thus, when the focus and tracking lens assembly begins a pass over the record medium after having been removed from the record medium, if the focus and tracking lens assembly has been allowed to wander, the time taken ro obtain proper focus and tracking of the optical beam on the track of information to be retrieved is increased. This of course, results in the loss of recorded information when reading a track of recorded information. Moreover, in these systems, if the arcuate shape of the track of information to be read from the record medium is not the same as the arc followed by the objective lens assembly over the record medium, the tracking actuator must compensate for the differences to maintain the optical beam on the track of data. However, since the range of the tracking actuator is limited, if the arc traversed by the lens assembly over the record medium is sufficiently different than the shape of the arcuate track of recorded information to be read, the arcuate track of information cannot be completely retrieved. Attempts have been made to correct this problem. In particular, U.S. Patent No. 4,416,001 to Ackerman et al. shows an apparatus for optically reading digital data recorded in spaced arcuate tracks on a data carrier. When information is to be read from the medium, the medium is adjusted in position until the arc of the first track aligns with the arc traversed by the objective lens over the medium. Once this has been done, all tracking functions in the apparatus are de¬ activated, since the device assumes that all tracks recorded on the medium have the same centre of curvature as the first track and that the locus of the centre of curvature of each recorded track lie along the centre line of the record medium. When this is complete, the objective lens and light source are rotated so that the optical beam traces an arcuate path across the medium. Once one of the tracks has been read, the medium is incrementally advanced so that the next track recorded on the medium may be read. However, a problem exists in this device in that once the initial skew between the first track and the arc followed by the objective lens has been removed and the tracking function has been de-activated, if any of the other tracks of recorded information do not have SUBSTITUT SHE a centre of curvature identical to the first track or if the centre of curvature of a recorded track does not lie on the centre line of the record medium, these tracks of recorded information cannot be read properly. This is due to the fact that the arc followed by the optical beam across the data carrier will not be completely aligned with these tracks. Moreover, since the tracking functions in the system are non-functional, the difficulties cannot be compensated. Accordingly, there is a need for a novel optical system. It is therefore an object of the present invention to obviate or mitigate the above-mentioned disadvantages. DISCLOSURE OF INVENTION According to the present invention there is provided an optical system comprising: a light source for generating an optical beam; support means movable about an axis and supporting at least one lens means spaced from said axis; drive means for moving said support means about said axis to pass said lens means in an arcuate path across a record medium; means for directing said optical beam through said lens means so that said optical beam is focussed on said record medium during the pass of said lens means across said record medium; detection means for detecting whether the pass of said lens means across said record medium is aligned with a track of information to be read from said record medium; SUBSTITUTE ET adjustment means responsive to said detection means for adjusting the position of said record medium when the pass of the lens means is not aligned with said track so that the next pass of said lens means across said record medium is substantially aligned with said track; and data recovery means receiving said optical beam after said optical beam impinges on said medium and recovering said information therefrom. A method of retrieving recorded information from a record medium is also provided. In another aspect of the present invention there is provided an optical recording and/or playback system comprising: a light source for generating an optical beam; support means for supporting at least one lens means; drive means for moving said support means to pass intermittently said lens means across a record medium; means for directing said optical beam through said lens means to impinge on said record medium; focus means receiving said optical beam prior to said optical beam impinging on said medium; detection means receiving said optical beam after said beam has impinged on said record medium and detecting whether said optical beam is focussed on said medium, said detection means generating focus error signals when said optical beam is not focussed on said medium; and focus correction means operable in a first mode when said lens means is passing across said record medium and a second mode when said lens means is remote from said record medium, in said first mode, said focus correction means being responsive to said focus error signals and being operable to adjust said focus means to maintain the focus of said optical beam thereon and in said second mode, said focus correction means maintaining said focus means in a predetermined position. In still yet another aspect of the present invention there is provided an optical recording and/or playback system comprising: a light source for generating an optical beam; support means for supporting at least one lens means; drive means for moving said support means to pass intermittently, said lens means across a record medium; means for directing said optical beam through said lens means to impinge on said record medium; focus means receiving said optical beam prior to said beam impinging on said medium; detection means receiving said optical beam after said beam has impinged on said record medium and detecting whether said optical beam is focussed on said medium, said detection means generating focus error signals when said beam is not focussed on said record medium; focus correction means operable in a first mode when said lens means is passing across said record medium and a second mode when said optical beam is remote from said record medium, in said first mode, said correction means being responsive to focus error signals and being operable to adjust said focus means to maintain the focus of said beam thereon and in said second mode, said correction means detecting the distance of said lens means from said medium as said lens means approaches said medium, and moving said focus means from a predetermined position to another position at a constant rate commencing at a predetermined distance from said medium; said detection means further detecting when said optical beam attains focus on said medium after passing over data recorded on • siad record medium and providing signals to said focus correction means so that the predetermined distance from said medium is adjusted for the next pass of said lens means across said medium so that focus of said optical beam on said medium is attained more quickly during a successive pass of said lens means across said record medium so that focus is obtained prior to passing over data recorded on said record medium.. In still yet another aspect of the present invention there is provided an optical recording and/or playback system comprising: a light source for generating an optical read beam in a read mode and an optical write beam in a record mode; focussing and directing means for receiving and directing said optical beam along a path on a record medium to record a track of information thereon, said record medium having at least one track of information recorded thereon; detection means receiving said optical beam after said beam has impinged on said record medium in said read mode and determining the beginning and ending positions of said track; processing means communicating with said detection means, said processing means interpolating between said beginning and ending positions using a typical path followed by said beam across said record medium and generating therefrom profiling signals; and correction means for altering the position of said optical beam on said medium, said correction means being responsive to said profiling signals for altering the path of said optical beam in said write mode so that the track of information to be recorded thereon represents an attempt to replicate the shape of the previously recorded track. Preferably in the first aspect of the invention, the record medium is pivoted in response to signals generated by the detection means to alter the position of the record medium so that the path traced by the lens means across the record medium is substantially aligned with the recorded arcuate track of information to be read from the record medium. This is achieved by pivoting the record medium so that a line parallel to the longitudinal axis of the record medium passing through the centre of curvature of the track to be read, intersects the axis of movement of the support means. The present device provides advantages in that since any differences between the arc traversed by the lens means over the record medium and the arc of the tracks recorded on the record medium can be substantially removed prior to reading the tracks of information, all recorded tracks are maintained within the limits of the focus and tracking servo-units and actuators. This allows substantially all of the recorded information to be efficiently recovered. Also, since the system does not remove card skew in the system but rather corrects eccentricity errors, the need for providing a very accurate record medium edge reference is removed. Moreover, since the tracks of information recorded on the record medium are typically recorded having a similar arc, correction to remove differences between the arc traversed by the lens means and the recorded tracks of information to be recovered need not be performed for each recorded track of information on the record medium. Rather, the adjustment of the record medium needs to be performed only when the shape of the track and the arcuate path of the optical beam differ by an amount that can not be compensated for by the tracking and focus servo-units and actuators. However, the present design permits the position of the record medium to be altered for the reading of each track recorded on the record medium if required. BRIEF DESCRIPTION OF DRAWINGS Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a side view of an optical system including an eccentricity corrector; Figure 2a is a partial cut-away perspective view of an optical scanner drum used in the system illustrated in Figure 1; Figure 2b is a top plan view of a record medium; Figure 3 is a block diagram of the system shown in Figure 1; Figure 4a is a block circuit diagram of a component of the diagram illustrated in Figure 3; Figure 4b is a sectional view of a portion of the system shown in Figure 1; Figure 5 is a block circuit diagram of another component of the diagram illustrated in Figure 3; Figure 6 is a block.diagram of yet another component of the diagram shown in Figure 3; Figures 7a and 7b illustrate a position sensor and a response curve therefor, used in the system illustrated in Figure 1; Figures 8a and 8b illustrate signal level curves for components of the system shown in Figure 1; Figure 9 is a top view of another portion of the system shown in Figure 1; Figure 10 is a perspective view of the portion shown in Figure 9; Figures 11a and lib are top views illustrating the operation of the portion shown in Figure 9; and Figures 12a and 12b are illustrations showing further the operation of the portion shown in Figure 9. BEST MODES FOR CARRYING OUT THE INVENTION Referring to the Figures, an optical recording and/or playback system 10 is shown. The system 10 includes a carriage 14 disposed within a housing 12. The carriage 14 supports one side of a reflective record medium 16. The record medium 16 is capable of allowing arcuate tracks 16a of digital information to be recorded thereon or to be read therefrom using an optical beam 18. A carriage actuator 20 in the form of a stepper motor is in communication with the carriage 14 and functions in a known manner to alter incrementally by a predetermined distance, the position of the record medium 16 along an axis "y". This allows successive tracks of information 16a recorded on the record medium 16 to be scanned by the optical beam 18 and allows tracks of information to be recorded on the record medium. The carriage actuator 20 receives input signals from a card motion function 22 in the form of a motor driver. The function 22 receives input signals from a carriage position sensor 24 that is also in communication with the carriage 14. This allows the system 10 to determine the position of the carriage 14 along the axis y. Since the operation of these components is well known in the art, details of the operation thereof will not be described herein. A second carriage actuator 26 in the form of a magnetizable coil 26a is provided on the carriage 14 and operates to move the carriage 14 in an x-y or recording plane as will be described herein. The carriage actuator 26 is also in communication with an eccentricity correction function 23. A second carriage position sensor 28 is similarly in communication with the carriage 14 and with the eccentricity correction function 23. The input signals received by the function 23 from the position sensor 28 allow the system 10 to determine the position of the ' carriage 14 in the recording plane. A scanner drum 32 is disposed on the other side of the record medium 16 and rotates about an axis 34. A single lens assembly 36 including an objective BSTITUTE SHEET lens and a focus and tracking lens is disposed on the drum 32 and is spaced from the axis 34. The lens assembly 36 intermittently traces an arcuate path over the record medium 16 as the scanner drum 32 rotates. An optical read/write laser source 38 is mounted below the lens assembly 36 and generates the optical beam 18. The intensity of the optical beam generated by the laser source 38 is dependant on whether information is to be recorded on the record medium 16 or read from the record medium. The laser source 38 and the lens assembly 36 are aligned so that the optical beam 18 follows a path through the lens assembly along the optical axis of the lens assembly towards the record medium 16 that is substantially parallel to the axis 34. A photo-detector array 40 is also included and comprises six photo-detectors arranged in the manner shown in Figures 4a, 5 and 6, four 40a of which are used for focus and two 40b of which are used for tracking. The array 40 is disposed adjacent the laser source 38 and receives the reflected optical beam 18' from the record medium 16. Each of the photo-detectors 40a, 40b generates electrical signals upon reception of the optical beam 18' after it has been reflected from the record medium 16. The electrical signals generated by the photo-detectors 40a, 40b are conveyed to an optical pre-amplifier 41. The pre-amplifier 41 is better illustrated in Figure 4a and is of a typical design. The pre-amplifier comprises a pair of subtractors 41a and 41b formed from differential amplifiers (diff-amps), three adders 41c, 41d and 41e respectively and two peak holders 41f and 41g. The pre-amplifier receives the output of the SUBSTITUTE SHEET photo-detectors and generates, a tracking error voltage signal TE, a focus error voltage signal FE as well as retrieves a modulated signal HF which includes the high frequency digital data that are recorded in the form of the tracks 16a on the record medium 16. The focus error signal FE is generated by adding separately the output of the diagonally located photo-detectors 40a via adders 41d and 41e. The two resulting sum signals are applied to a peak holder 41f and then subtracted via subtractor 41b to form the focus error signal FE. The tracking error signal TE is generated by passing the output signals of the photo- detectors 40b through a peak holder 41g and then subtracting the output signals via subtractor 41a. The modulated HF signal is recovered by taking the sum of the output signals generated by each photo-detector 40a, 40b using adder 41c. Since the operation of the pre¬ amplifier 41 is known in the art, the operation thereof will not be discussed in any further detail herein. A focus servo-unit 42 receives the focus error signals FE from the pre-amplifier 41 as well as timing and predetermined operation signals from a microprocessor 52 via a digital to analog convertor (DAC) 43. The focus servo-unit 42 generates focus correction signals from the focus error and predetermined operation signals and conveys the correction signals to a focus actuator 46. The focus actuator 46 is disposed adjacent the lens assembly 36 and comprises a pair of coils that are coupled to the focus and tracking lens. The coils function in a known manner to move the focus and tracking lens axially with respect to the optical beam 18 when energized by the focus correction signals to allow the focal point of the optical beam 18 to be adjusted. A tracking servo-unit 44 receives the tracking error signals TE from the pre-amplifier 41 along with timing and predetermined operation signals from a second DAC 45. The tracking servo-unit 44 generates tracking correction signals from the tracking error and predetermined operation signals and conveys the correction signals to a tracking actuator 48. The tracking actuator 48 is also disposed adjacent the lens assembly 36 and comprises a pair of coils that are coupled to the focus and tracking lens. These coils function in a known manner to move the lens perpendicular to the axis of the optical beam 18 when energized by the tracking correction signals. A focus position sensor 49 is also provided and communicates with the focus servo-unit 42 and the lens assembly 36. The sensor 49 provides information to the focus servo-unit 42 representing the position of the focus and tracking lens in the axial direction with respect to a default position as will be described herein. A tracking position sensor 51 is also provided and communicates with the tracking servo-unit 44 and the focus and tracking lens. The-tracking position sensor 51 provides information representing the position of the lens with respect to a default position in a direction transverse to the optical beam 18. Since the position sensor 51 functions in a similar manner to the focus position sensor 49, the position sensor 51 will not be described in detail herein. An automatic gain control circuit (AGC) and phase lock loop (PLL) 50 receives the modulated HF signal outputted by amplifier 41c so that the recorded digital information can be retrieved when scanned by the optical beam 18. The digital output of the AGC circuit and PLL 50 is applied to the microprocessor 52 before being optically transmitted along an optical data link 54. As is well known in the art, the optical data link includes electrical to optical and optical to electrical converters as well as a glass fibre. The microprocessor 52 is also connected to the DAC 45 and provides the digital timing and predetermined operation signals thereto so that they may be converted into an analog form prior to being applied to the tracking servo-unit 44. A laser drive circuit 56 also receives input signals from the microprocessor 52 and provides power to the laser source 38 as well a biasing voltage for the photo-detectors 40a, 40b in response to these signals. The microprocessor 52 also receives input signals from a timing logic function 58a so that the digital information outputted and received by the microprocessor 52 is clocked at appropriate intervals. A rotary switching transformer 60 is positioned within an annular recess formed in the scanner drum 32 and surrounds the optical data link 54. The rotary switching transformer 60 is connected to a switching power supply 62 at its primary terminals and to the various components disposed on the drum 32 at its secondary terminal. The transformer 60 provides isolated power to the components on the drum 32 and is used to reduce noise in the system 10 typically encountered when using systems implementing brushes to convey electrical power to a rotating component. The rotary transformer 60 is better illustrated in Figure 4b and as can be seen, the transformer secondary 60a comprises a magnetic core 60b surrounded by coils 60c. The secondary 60a is mounted on the drum 32 and is rotatable therewith. The transformer primary 60d is stationary and is disposed directly below the secondary 60a. The primary 60d has a similar configuration as the secondary and receives a 12 v supply voltage across its terminals from the power supply 62. The secondary 60a in turn is energized when the primary is energized even while the drum 32 rotates to allow the components disposed on the drum 32 to be supplied with power. A brushless DC motor 64 is also provided on the scanner drum 32 and functions to rotate the scanner drum 32 at a substantially constant speed. The motor 64 is driven by a motor driver 66 that receives information from a second microprocessor 68 as well as from an encoder 70. The microprocessor 68 is also in communication with the optical data link 54 so that information can be transferred between the two microprocessors 68 and 52 and with the eccentricity correction and card motion functions 22 and 23 respectively. An index mark 72 as well as a plurality of closely spaced position marks are located on the outer surface of the scanner drum 32. The encoder 70 which is spaced from the scanner drum 32 detects these marks via T a light source, light detector and counter (not shown) and provides signals to a timing logic function 58b. The timing function 58b provides clocking information to the microprocessor 68 in response to the signals received from the encoder 70. The timing function 58b also communicates with the timing function 58a disposed on the scanner drum 32 so that the components disposed on the drum and those located remote from the drum 32 remain in sync. A read/write function 74 is in communication with the microprocessor 68. The function 74 provides instructions to the microprocessor 68 that are used by the system 10 to control the operation of the laser source 38. A small computer system interface (SCSI) 76 communicates with the microprocessor 68 and allows data transfer between the microprocessor 68 and a user micro¬ computer 78. Referring now to Figure 5, the focus servo- unit is better illustrated. As can be seen, the focus servo-unit includes a compensator 42c receiving the output of diff-amp or subtractor 41b. A focus-servo control logic function 42d also receives the output of the diff-amp 41b. A gain control circuit 42e receives the output of the compensator 42c and provides an input signal to an analog switch 42f. The control logic function 42d receives input timing signals from the microprocessor 52 via the DAC 43 over control lines 42g. The control logic function provides two signals to a ramp generator 42h, a control signal to two other analog switches 42i and 42j respectively as well as a signal to a sample and hold circuit 42k. The output of the ramp generator 42h is conveyed to the analog switch 42i. The sample and hold circuit 42k also receives an input signal from a diff-amp 421 that is connected to the focus position sensor 49. The sample and hold circuit 42k provide an output signal to the analog switch 42j representing the position of the focus and tracking lens at the best focus position during its previous pass over the record medium. The output signal conveyed by the analog switch 42j is combined with a focus demand signal that is supplied to a focus demand conductor 42m by the microprocessor 52 via the DAC 43. The resulting signal in turn is fed to a summing block 42n. The summing block 42n also receives the output of analog switch 42i and the inverted output of analog switch 42f and forms a resultant signal equivalent to the sum of the three input signals. The resultant signal is then applied to a power amplifier 42o wherein an appropriate correction signal is formed which is applied to the focus actuator 46. The focus position sensor 49 is illustrated in Figures 7a and 7b and as can be seen includes a light source 49a mounted on the scanner drum 32 adjacent the lens assembly 36. A mirror 49b is secured to the lens assembly 36 and reflects a light beam 49c that is generated by the light source 49a towards a set of photo-detectors 49d. The photo-detectors 49d provide an output signal having a linear response as shown in Figure 7b. The magnitude of the output signal is determined by the axial movement of the focus and tracking lens from its rest or default position. As mentioned previously, the output signals from the position sensor 49 are conveyed to the focus servo-unit 42 to allow the position of the focus and tracking lens to be fixed in the axial direction when the objective lens assembly 36 is remote from the record medium 16. Although not shown, the tracking position sensor 51 includes the same components as the focus position sensor. The output from the photo-detectors of the tracking position sensor are conveyed to the tracking servo-unit 44 to allow the focus and tracking lens to be fixed in the transverse direction when the lens assembly 36 is off the record medium and to allow the lens to be pre-positioned when it is desired to write tracks of data as will be described. Referring now to Figure 6, the tracking servo- unit is better illustrated. The servo-unit 44 includes a compensator 44c which receives the output signal from the diff-amp 41a. The output of the compensator 44c is applied to a gain control circuit 44d. The output of the gain control circuit 44d is conveyed to an analog switch 44e, the position of which is controlled by the output of a control logic block 44f. The control logic block 44f receives input signals from the microprocessor 52 via the DAC 45 and the control lines 44g and provides output signals to a plurality of components in the tracking servo-unit 44. A differential amplifier 44h is in communication with the tracking position sensor 51 and provides an output signal to a sample and hold circuit 44i as well as to an ac-decoupling function 44j in the form of a capacitor. The sample and hold circuit 44i which receives the output of the control logic 44f provides an output signal that is received by another diff-amp 44k. The diff-amp 44k also receives the output signal of diff-amp 44h and generates a difference signal that is conveyed to an analog switch 441. Similarly, the position of the analog switch 441 is determined by signals received from the control logic 44f. A compensator 44m receives the ac-decoupled output of the diff-amp 44h and provides an output signal to a gain control circuit 44n. The output of the gain control circuit 44n is conveyed to an analog switch 44o that is also controlled by the control logic 44f. An integrator 44ρ receives a signal from the control logic 44f along with guidance profile signals supplied to conductor 44q by the microprocessor 52 via the DAC 45. The output of the integrator 44p is conveyed to a gain control circuit 44r. The output of the gain control circuit 44r is also applied to an analog switch 44s which is similarly controlled by the control logic 44f. The output of analog switches 441 and 44s are combined with a tracking demand signal supplied to a conductor 44t from the microprocessor via the DAC 45 and applied to one input of a summing block 44u. The output of analog switch 44o and the inverted output of analog switch 44e are also supplied to the summing block 44u. The signal outputted by the summing block 44u is conveyed to a power amplifier 44v wherein it is boosted to form the tracking correction signal. The tracking correction signal is then applied to the tracking actuator 48. Referring now to Figures 9 and 10 ,the card carriage 14 is better illustrated. As can be seen, the carriage 14 comprises inner and outer frame assemblies 14a and 14b respectively. The outer assembly 14b includes a slot 14c for receiving the record medium 16 TE SHEET and is in communication with the stepper motor 20 and sensor 24 so that it can be displaced incrementally along the axis y. The inner frame assembly 14a is disposed within the outer frame assembly 14b and can be pivoted within the outer assembly in the x-y or recording plane. The inner assembly 14a is also provided with a slot 14c to allow the record medium 16 to be supported thereby. The inner assembly 14a is pivotally mounted to the outer assembly 14b via a cross-flat spring 14e. The pivot structure is designed to prevent undesired movement of the inner assembly 14a in the direction of the x axis and to provide low friction when adjusting the position of the record medium 16 to correct eccentricity errors. The mass of the inner assembly 14a is also reduced to enhance response time when moving the inner assembly, the response time typically being less than 100ms with a static relative position error of less than +/-2μm (microns). A bar magnet 14f is disposed on the inner assembly and is in communication with the carriage actuator 26. The moving coil 26a of the actuator 26 provides the eccentricity adjustment torque and is capable of providing a peak force of approximately 8.0 oz per inch on the inner assembly 14a. This permits a maximum angle of pivot of the inner assembly 14a of approximately 1° which is suitable for removing eccentricity between the recorded tracks and the path followed by the lens assembly-36 over the record medium 16. An LED 14g is mounted on the inner assembly and communicates with the carriage sensor 28. The carriage position sensor 28 includes a feedback circuit comprising an optical sensor 28a in the form of a bi-cell detector which communicates with the LED 14g. The optical sensor 28a is shielded from all outside background light. The output of the detector 28a is applied to a diff-amp 28b which provides a zero volt output signal if the inner assembly 14a is in its rest position (i.e. the pivot angle 0 is 0° ). However, as the inner assembly 14a is pivoted, the diff- amp 28b generates an output signal having a polarity dependant on the direction of the pivot and a magnitude dependant on the angle of the pivot. The output of the diff-amp 28b is applied to the eccentricity correction function 23 so that the position of the inner assembly 14a can be monitored. The function 23 includes a DAC 23a connected between the microprocessor 68 and a summing block 23b. The summing block 23b also receives the output signal generated by the diff-amp 28b and provides an output signal that is conveyed to a linear motor driver and amplifier circuit 23c. The amplifier circuit 23c conditions the signal received from the summing block and conveys it to the coil 26a of the actuator 26 causing it to move. As the coil 26a moves, the bar magnet 14f follows the movement of the coil which in turn causes the inner assembly 14a to pivot within the outer assembly 14b about the spring 14e. The diff-amp 28b and the amplifier 23c implement a closed-loop position control system to allow position control of the carriage 14 to be maintained accurately. The operation of the system 10 will now be described with reference to the Figures and with an emphasis on the operation of the eccentricity correction function 23 and carriage 14. When it is desired to read a track 16a of information from the record medium 16 that is supported by the carriage 14, the system 10 is powered up. Thereafter, the microprocessor 68 conveys information to the motor driver 66 which in turn energizes the motor 64 so that the drum 32 is rotated at a substantially constant speed. As the drum rotates, the encoder 70 detects the index mark and position marks and counts the marks so that the position of the laser source 38 and lens assembly 36 with respect to the carriage 14 can be determined by the microprocessor 68. The encoder 70 while doing this also provides a feedback signal to the motor driver 66 so that the speed of rotation of the drum 32 can be monitored and maintained. The encoder 70 also provides clock pulses to the timing generator 58b. The timing generator 58b in turn uses the clock pulses to provide timing pulses to the microprocessor 68 so that the information therein is clocked properly as well as to provide timing signals to the timing generator 58a on the drum 32. At the same time, the power supply 62 is also energized via the 12 v supply voltage so that the supply voltage can be conveyed to the components disposed on the drum 32 via the transformer 60. After completing this, the read/write function 74 is set to the read mode and appropriate information is conveyed to the microprocessor 68. The microprocessor 68 in turn conveys the information along the optical data link 54 to the microprocessor 52. The microprocessor 52 upon receiving this information supplies control signals to the laser drive circuit 56 which conditions the laser source 38 to generate an optical beam 18 having an intensity suitable for reading digital information from the record medium 16. If it is desired to write information on the record medium 16, the read/write function 74 is set to a write mode. Similarly, control information is conveyed to the laser drive circuit 56 in the same manner described above. The drive circuit 56 in this mode conditions the laser source 38 to generate an optical beam having an appropriate intensity for recording digital information on the record medium. Once the above steps are complete, the system 10 is operative and an eccentricity correction technique is implemented before any of the recorded information on the record medium 16 is retrieved. This ensures that the entire track or tracks to be read fall within the focus and tracking limits of the actuators 46, 48. Firstly, as the laser source 38 and lens assembly 36 commence to trace an accurate path across the record medium 16, the timing generator 58a provides information to the microprocessor 52 in response to the output signals of the encoder 70. The microprocessor 52 supplies information to the laser drive circuit to enable the laser drive circuit 56 so that an optical read beam 18 is generated by the laser source 38. The optical beam emerges from source 38 and passes through the lens assembly 36 prior to impinging on the record medium 16. As the lens assembly 36 passes across the record medium 16 to direct the optical beam 18 thereon, the photo-detectors 40 receive the reflected beam 18' from the record medium. The pre-amplifier 41 receives the output of the photo-detectors and in turn generates focus and tracking error signals in the manner previously described. The error signals are then applied to the respective servo-unit so that focus and tracking correction signals can be generated. The correction signals generated by the servo-units in response to the error signals are applied to the respective actuator 46, 48 so that the position of the focus and tracking lens can be adjusted in an attempt to maintain the optical beam 18 focussed on the track 16a of recorded information. The focus and tracking processes will be described in more detail herein. While this occurs, the microprocessor 52 monitors the output of the tracking servo-unit 44 to detect when the optical beam 18 crosses a recorded track 16a of information. Once this is done, the output of the timing generator 58a is monitored so that the rotation of the drum 32 after the first track crossing can be determined. If it is found that the optical beam 18 does not cross a second track during its pass over the record medium 16 yet remains locked onto a single track 16a, the carriage 14 remains stationary and the system re-reads the track so that the recorded digital data can be recovered. If the first track is not the desired track to be read, the microprocessor 68 provides commands to the motor driver of card motion function 22 which in turn causes the stepper motor 20 to advance the carriage 14 and hence the record medium 16 along the axis y until the desired track to be read is located above the arc traced by the lens assembly 36 over the record medium. If however, the optical beam 18 crosses a second track 16a during the above-mentioned step, the second track crossing is detected and the angle of rotation of the drum 32 that occurred between the two track crossings is determined using the output of the timing generator 58a. The track crossings are relative timing signals which are converted to angular information by suitable software as will be described. By knowing the radius of the scanner drum 32, the distance between the two track crossings and the track pitch can be determined from this angular information. Once these measurements are known, the angle of rotation 0 of the record medium 16 necessary to bring the arcuate shape of the first track 16a into alignment with the arc traversed by the lens assembly 36 over the medium can be determined. The necessary angle of rotation of the record medium 16 to remove eccentricity error is determined by taking the arc sine of the track pitch divided by the distance between the two detected track crossings as is shown in Figures 12a and 12b. Appendix 1 includes the source code of the program stored in the microprocessor 68 which performs the calculations to determine the track pitch, the distance between the two track crossings and the required pivot angle 0 of the record medium 16. Once the angle of rotation 0 of the record medium has been determined, the microprocessor 68 conveys this information to the eccentricity function 23. The DAC 23a therein converts the digital information received from the microprocessor 68 into analog form and applies the signal to the amplifier 23c. S B The amplifier boosts the signal and conveys it to the coil 26a thereby energizing it so that it moves across the assembly 14a. As the coil 26a moves, the bar magnet 14f affixed to the assembly 14a moves therewith, causing the inner assembly 14a to pivot about the spring 14e. The signal supplied to the coil and the resulting pivot of the inner assembly 14a and record medium 16 are of a magnitude to position the centre line C of the record medium 16 so that it passes through the axis of rotation 34 of the scanner drum 32 as illustrated in Figures 11a and lib if the centre of curvature of the track is positioned on the centre line of the medium. If the centre of curvature is displaced from the centre line of the record medium, the medium is pivoted through the angle 0 so that a line parallel to the centre line of the card passing through the centre of curvature of the track intersects the axis of rotation of the drum 32. When the assembly 14a pivots to its desired position, the bi-cell detector 28a applies a- signal to one of the two terminal of the diff-amp 28b, the terminal receiving the signal being dependant on the direction of the pivot of the assembly 14a. The diff- amp 28b in turn supplies a signal opposite in polarity to the signal generated by the DAC 23a but equal in magnitude which is received at the summing block 23b. The diff-amp signal negates the DAC output so that the input signal to the amplifier 23c goes to zero once the inner assembly 14a of the carriage has been pivoted by the desired angle 0. This in turn removes the linear drive bias supplied to the coil 26a to prevent any further movement thereof. However, the position of the inner assembly is maintained until it is desired to re¬ orient the position of the inner assembly. Once this is done, substantially all differences between the arc traced by the lens assembly across the record medium 16 and the shape of the track 16a of information to be read are removed. Thereafter, if necessary, the microprocessor 68 supplies signals to the card motion function 22 so that the stepper motor is actuated to move the carriage 14 along the axis y so that the track to be read is disposed directly above the lens assembly when it begins its next pass over the record medium. Under worst conditions, readjustment of the record medium position needs to be made approximately every 565μm of travel of the carriage 14 along the axis y to keep the record medium oriented so that the tracks of information thereon can be recovered. However, this operation can be performed prior to reading each track from the medium. After the position of the record medium 16 has been adjusted in the manner described above, the system 10 is able to recover the recorded data as will now be described herein below with particular emphasis on the operation of the focus and tracking servo-units. When recovering or writing data from or on the record medium 16, the system 10 employs three functions, these being focussing, profiling and tracking. The focus and tracking servo-units 42,44 are used to perform focus and tracking functions. The position sensors 49 and 51 are employed to compliment the focus and tracking functions. The profiling function is used to position the lens assembly 36 and in particular, the focus and tracking lens in pre-set positions when information is to be recorded on the record medium 16 so that the optical beam impinges on the record medium in a desired manner. Typically, the profiling is used so that the optical beam 18 is directed onto the medium 16 such that it follows a path acorss the record medium similar in shape to the shape of the last written track on the medium. This of course, increases memory storage capabilities in that the profiling function allows the track to track separation distance to be reduced. Since the lens assembly and laser source are removed from the record medium during a portion of the rotation of the scanner drum, the focus servo-unit 42 does not receive any focus error signals from the pre¬ amplifier 41 while it is remote from the record medium 16. This places the focus servo-unit 42 in an open-loop condition, since feedback from the pre-amplifier is removed. However, the focus servo-unit must assume a closed-loop condition as soon as the optical beam 18 hits the leading edge of the record medium 16 and must re-gain focus on the record medium 16 quickly if recorded information on the medium is to be retrieved quickly. This on/off or equivalently closing and opening of the focus servo-unit 42 dictates the hardware and software requirements thereof. When the lens assembly 36 is in an off medium position, the focus position sensor 49 output signals are used. In particular, when the assembly 36 is remote from the record medium 16 and focus error signals are not received from the pre-amplifier 41, a closed-loop condition is formed between the servo-unit 42, focus actuator 46, lens assembly 36 and the position sensor 49. SUBSTIT In particular, when the assembly 36 is remote from the medium 16, the microprocessor 52 detects this condition from the output of the timing function 58a. The microprocessor 52 in turn conveys signals to the control logic 42d via the DAC 43 and control lines 42g. The logic 42d in turn opens analog switches 42f and 42i and closes analog switch 42j. At this time, the logic 42d provides instructions to the sample and hold circuit 42k causing it to supply the signal to the summing block 42n via the switch 42j. The signal stored in the sample and hold circuit represents the position of the tracking and focus lens at its best focus position (FE=0) during the previous pass over the record medium. The best focus signal is combined with a default assembly position demand signal, which is typically zero volts, received from the microprocessor 52 via DAC 43 and conductor 42m. The position demand signal is set at a level so that the objective lens assembly 36 assumes a default position when no other signals are received by the servo-unit 42. The amplifier 42o receives the output signal generated by the summing block 42n and boosts the signal. The boosted signal in turn is conveyed to the focus actuator 46 so that the focus and tracking lens is moved axially with respect to the path of the optical beam 18 to the best focus position during its off-medium travel. When the lens assembly 36 approaches the record medium, the microprocessor 52 is supplied with appropriate timing information from the encoder 70 and timing function 58a. When the assembly reaches a predetermined number of encoder marks from the edge of the medium, the switch 42j is opened and the switch 42i is closed via the logic function 42d. At this time, the generator 42h is caused to output a ramp voltage which is conveyed to the summing block 42n via the switch 42i. The ramp voltage is combined with the focus demand signal and then conveyed to the amplifier 42o. The amplifier 42o in turn supplies the amplified ramp voltage to the focus actuator 46 which moves the focus and tracking lens axially with respect to the optical beam 18 at a constant rate. While this occurs, the focus error signal generated by the pre-amplifier 41 is monitored and the position sensor 49 output signal is sampled. When the focus error signal reaches the best focus position as shown in Figure 7b, the position of focus and tracking lens at the best focus position is stored in the sample of hold circuit 42k. This best focus position signal is generated by the diff-amp in response to the output of the photo-detectors included in the focus position sensor. If the optical beam 18 is not focussed properly on the record medium 16 prior to the beginning of a track to be read, the track 16a to be recovered is re-read. While the assembly 36 is removed from the medium, the system uses the new best focus position signal stored in the sample and hold circuit 42k to move the assembly 36 to a position closer to the optimum focus position while it is removed from the medium 16. When the assembly 36 approaches the medium 16 again, the assembly 36 is driven by the ramp voltage. However, the ramp voltage is applied to the summing block when the assembly 36 is in a position closer to the edge of the medium so that focus is achieved more quickly and prior SHEET to the beginning of a track to be read. Hence, subsequent scans of the optical beam 18 on the medium 16 require shorter times to ramp to the best focus position. This allows the focus of the optical beam 18 on the medium 16 to be achieved faster and allows all of the information recorded on the track to be recovered. It should be apparent that the ramp voltage is applied to the assembly 36 in a manner so that the assembly does not achieve the best focus position for the track to be read prior to the optical beam impinging on the record medium. This ensures that the best focus position is attained as the assembly is moved by the ramp voltage and while the assembly is positioned over the record medium. Once a track is to be read and focus of the optical beam 18 on the record medium 16 can be achieved using the ramp voltage and best focus pre-positioning quickly enough to recover all of the recorded data track, the focus servo-unit 42 relies on the focus error signals generated by the pre-amplifier 41 to maintain proper focus of the optical beam 18 on the medium 16. Accordingly, as soon as the optical beam impinges on the record medium 16, the switch 42f is closed and the switches 42i and 42j are opened. While focussing of the optical beam on the medium is being performed in the above-mentioned manner, a somewhat similar process is performed by the tracking servo-unit 44. As the optical beam 18 traces across the record medium 16, the tracking error signals generated by the pre-amplifier 41 are applied to the compensator 44c which compensates for the frequency response of the circuitry. The signals are then applied to the gain control circuit 44d before being applied to the summing block 44u via analog switch 44e. While a track of information is being read, the control logic 44f opens the analog switch 44s to prevent the profiling signal applied to conductor 44q from being supplied to the summing block 44u. Similar to the focus servo-unit, a demand position signal is supplied to conductor 44t so that the focus and tracking lens is maintained at a default position when the other inputs applied to the summing block 44u go to zero. The tracking position sensor 51 monitors the position of the focus and tracking lens in the tracking direction and provides an output signal which is received by the differential amplifier 44h. The diff- amp 44h provides an output signal to the sample hold circuit 44i as well as to the differential amplifier 44k. The differential amplifier 44k receives the output of the sample and hold circuit 44i and provides an output signal equivalent to the difference between successive positions of the focus and tracking lens in the tracking direction between two clock pulses. This difference signal is conveyed to the summing block 44u via the analog switch 441 and is combined with the tracking demand signal when the assembly 36 is passing over the record medium. At this time, the control logic 44f provides a signal to analog switch 44o to prevent the ac-decoupled circuit from * providing control signals to the summing block 44u. Accordingly, when the optical beam 18 is scanning the medium 16, the tracking servo-unit 44 operates in a similar manner to the focus servo-unit to maintain the optical beam substantially on the track of ET information being read. The output of the differential amplifier 44k is used by the tracking servo-unit when the tracking error signal received from the pre¬ amplifier 41 goes to zero. This ensures that the tracking amplifier 44v is always supplied with an input signal having a magnitude other than zero volts. This input signal in turn is supplied to the tracking actuator 50. The tracking actuator 50 upon reception of this signal adjusts the position of the focus and tracking lens assembly 36 to maintain the optical beam 18 substantially on the recorded track of information even if the output of the pre-amplifier 41 goes to zero. This is due to the fact that since the differential amplifier 44k continuously provides a signal representing successive movements of the focus and tracking lens, the tracking actuator 50 is continuously supplied with a correction signal to cause the focus and tracking lens to dither close ' to the best tracking condition. This allows the system 10 to track the entire recorded track during the pass of the optical beam 18 across the record medium 16 without the focus and tracking lens from wandering. When the lens assembly 36 moves off the record medium 16, the analog switch 44s and the analog switch 44e are opened by the control logic 44f. The analog switch 441 is maintained in a closed position so that signals are continuously combined with the tracking demand signal and conveyed to the summing block 44u. The analog switch 44o is also closed so that the diff- amp output signal 44h is ac-decoupled via the capacitor 44j and conditioned prior to being supplied to the summing block 44u. The signal received by the summing block from the demand line and from the differential SUBSTITUTE SHEET amplifier 44k function to move the focus and tracking lens assembly 36 to its default position when the lens assembly 36 is removed from the medium 16 quickly. The output of the differential amplifier 44h which is decoupled is used to alter the frequency response of the actuator 50 so that the focus and tracking lens moves to its default position as determined by the demand signal and the differential amplifier 44k output signal quickly and with little oscillation. This ensures that the lens is in the proper tracking position when the lens assembly 36 begins its next pass across the record medium 16. Once focus and tracking of the optical beam is locked on the record medium, the output signals of the adder 41c which includes the recorded digital information are demodulated and digitized by the AGC and PLL circuit 50 prior to being conveyed to the microprocessor 52. From the microprocessor, the recorded data are optically transmitted across the link 54 and received by the microprocessor 68. The recovered digital information can then be transferred to the user computer 78 via the interface 76 so that the retrieved information may be used in other processes. It should be realized that if it is desired to read a different track of information from the record medium 16, the carriage actuator 20 is energized via the unit 22 so that the record medium 16 is advanced in the appropriate direction. This ensures that the optical beam 18 will impinge substantially on the proper recorded track of information to be retrieved. When it is desired to write a track of information on the record medium 16, the read/write function 74 is set accordingly so that the laser source 38 is properly conditioned. Prior to writing a track, the last written track of information on the medium is read in the manner described above. During this time, the position of the focus and tracking lens at the start and end of the track being read are monitored and used by the microprocessor 68. The microprocessor 68 interpolates between these lens positions using a typical arc followed by the assembly 36 over the medium and forms profiling signals. Once the profiling signals have been calculated, the carriage 14 is moved so that a track of information can be recorded on a blank portion of the medium. As the assembly 36 begins a pass over the record medium, the profiling signals are conveyed to the tracking servo-unit which alters the position of the tracking and focus lens to attempt to ensure that the track of information to be written is substantially the same shape as the last written track. During profiling, the analog switches 44e, 44o and 441 are opened to prevent signals from being supplied to the summing block 44u. The tracking demand signal is still supplied to the summing block 44u so that the focus and tracking lens assumes the default position. As the optical beam 18 begins to trace across the record medium 16, analog switch 44s is closed so that the summing block 44u is provided with a profile signal on conductor 44q via the integrator 44p and gain control circuit 44r. The combined signal received by the summing block 44u is supplied to the tracking power amplifier 44v. These signals outputted by the amplifier 44v are then conveyed to the tracking actuator 50 which T SHEET adjusts the position of the focus and tracking lens during the entire trace of the medium. This ensures that the track of information recorded on the medium is recorded having a shape determined by the tracking demand signal and the profile signal. When the lens assembly 36 is removed from the medium, the analog switch 44s is opened while the analog switch 44o remains closed. The focus tracking demand signal is also maintained so that the position of the focus and tracking lens is held at the default position. If it is desired to form profiling signals which more closely resemble the last written track, more information from the last written track can be used other than its starting and ending point. In particular, when reading the last written track, by closing the tracking loop via•switch 44e and monitoring the tracking error signal, a more precise profile can be generated to more closely resemble the shape of the track. This profile information can be stored in the microprocessor and used for track guidance during the write mode. Guidance for writing is required since the writing process is effectively open loop (i.e. the tracking function is de-activated). The profile information for the track to be written is received from the DAC 45 via the microprocessor 52 and is obtained from the profiling function. Although the system 10 has been described as having a single light source and lens assembly, it should be realized that a plurality of sources 38 and lens assemblies 36 can be disposed on the rotating scanner drum 32. In this manner, each successive optical head and objective lens assembly will pass over the record medium 16 to enable tracks of information to be recorded on the record medium and to be retrieved from the record medium at a faster rate. Although the focus servo-unit has been described as using both the ramp voltage generator and the sample and hold circuit, these two functions may be used independently. If desired, the ramp generator circuit can be eliminated. If this is done, the best focus position signal stored in the sample and hold circuit 42k is used in conjunction with the position sensor 49 to pre-position the assembly 36 during its off-medium condition. With the assembly maintained in a best focus position for the previous pass of the beam on the medium during its off medium condition, when the assembly begins a pass over the medium, the analog switch 42j is opened and the analog switch 42f is closed to allow the focus error signals to be used. Since the lens assembly is held in a best focus position during the off medium condition, the time taken to reach focus when the assembly reaches the medium is small. This is due to the fact that the best focus position does not deviate greatly between passes of the lens assembly over the medium. As an alternative, the sample and hold and focus position sensor circuit can be moved and the ramp generator circuit can be used by itself to obtain quick focus. When using the ramp generator circuit independently, the amount of rotation of the drum 32 is monitored from the time at which the analog switch 42i is closed and the assembly 36 is ramped. If it is determined that focus is achieved after the beam 18 passes over data recorded on the medium, the time at which the ramp is applied to the focus actuator 46 is altered so that the assembly attains the best focus position closer to the leading edge of the medium 16. This ensures that focus of the optical beam on the medium is attained quickly and prior to passing over any recorded data. It should be apparent to one of skill in the art that the present focus, tracking, profiling and eccentricity functions can be used in various optical systems implementing a discontinuous format systems including those described in U.S Patent No. 4,163,600 to Russell wherein the light source and focus and tracking lens are remote from the scanner drum. It should also be realized that various modifications and variations can be made to the present system without departing from the scope thereof as defined by the appended claims. Appendix A fdefine TGHPUsPerTrackCal 68 /Calibrated HPUs per track */ idefine TGMaxHPUs 4096 /* maximum dac value */ fdefine TGMAXCROSSINGS (TGMaxHPUs/TGHPUsPerTrackCal+1) /* defines for eccentricity correction */ /**** DEPENDENT ON RPH AND IDCAP MULTIPLIER ****/ /* Track pitch is 100000000 times larger than actual */ fdefine TGTracKPitch ((long) (3L * 100000000L)) /* The linear velocity of the drum is calculated as follows: */ /* drum speed in revolutions per us * the circumference of the */ /* drum ( 2*PI*47002 ) * 1000 */ fdef ine TGVlinear 1403 /* 285/60/1000/1000*47002*PI*2* 1000 */ /* The time be tween IDCAPs i s drum ci rcumf erence / 6 */ /* / number of DCAPs ( 142 ) / IDCAPs per DCAP ( 4 ) * TGVlinear */ /* / 1000. */ f def ine TGlDCAPTIHE 62 /* μsec per IDCAP at 285 rpm DCAP/4 */ BSTITUTE SHEET Appendix A cont'd /* TGGetTrackCrossings is used to get the total number of track crossing with the current beam path. Parameters: crossings - an array of IDCAP locations that track crossings have occurred at. Returns: The total number of track crossing encountered. »/ int TGGetTrackCrossings ( crossings ) short *crossings: { register int i: int TXcount: TGMode = TGSEEKMODE: while (ISCPOFFCard): /* wait for offcard signal */ /* now collect data on next scan */ while (SCPOFFCard): /* wait for on card */ while (iSCPFFCard): /* let dma collect data to evaluate */ /* filter out the area with track crossings */ /* criteria for track crossings */ /* FOR = 1. */ /* DSVOK = 1. */ /* XTRACK = 1. */ /* DSOK = 1. */ TXcount = 0: for (i=0: ((i<(SCAN * IDCAPFACTOR)) && (TXcount < TGMAX CROSSINGS)) : i /* insure conditions are correct for a track crossing */ if (((PP1F0K : PPlDSVOK : PPlXTRACK : PPlDSOK) & SOPSystemStatus [i] == (PP1F0K : PPlDSVOK : PPlXTRACK : PPlDSOK)) ( /* record track crossing event */ crossings[TXcount++] = 1: /* IDCAP crossing detected */ /* wait for this track crossing to go away */ for (:(i<(SCAN * IDCAPFACTOR)) && (((PP1F0K:PPlDSVOK:PPlXTRACK:PPlDSOK)&SCPSystemsStatus[i] -- (PP1F0K : PPlDSVOK : PPlXTRACK : PPlDSOK)): i++): return TXcount: /* return the number of track crossings found */ Appendix A cont ' d /* TGECDAngle is used to calculate the current tilt angle of the card. Parameters: ECD angle: pointer to a long to be used to store the calculated tilt angle * 10000. Returns: TG OK if successful. TG~BLANK if no data is encountered on the card. TG " ONETRACK if only 1 track crossing is encountered. */ int TGECDAngle ( ECD angle ) long *ECD angle: { register int i: int TXcount: short CrossPos[TGMAXCROSSINGS]: short TXTime: long DCrossings: TGNullPosition(): TXCount = TGGetTrackCrossings(δCrossPos[0]): if (TXcount == 0) : { /* must be a blank area of the card */ return TG BLANK: } if (TXcount == 1) ( /* something is wrong. There must be more. */ return TD ONETRACK: } /* Calculate the time between the first two track crossings */ TXTime = (CrossPosfl] - CrossPos[0]) * TGIDCAPTIME: /* calculate the distance between the two track crossings */ Dcrossings = (long) TXTime * (long) TGVlinear: /* with the distance and the time it is now possible to get the angle /* track pitch */ /* angle - sin -1 */ /* DCrossings */ /* note that sin (x) == x for small angles in radians */ /* Note that this angle is 10000 times greater than actual. */ /* This is done to facilitate integer math. */ *ECD_angle = TGTrackPitch /DCrossings return TG OK: SUBSTITUTE SHEET

Claims: WE CLAIM: 1. An optical system comprising: a light source for generating an optical beam; support means movable about an axis and supporting at least one lens means spaced from said axis; drive means for moving said support means about said axis to pass said lens means in an arcuate path across a record medium; means for directing said optical beam through said lens means so that said optical beam is focussed on said record medium during the pass of said lens means across said record medium; detection means for . detecting whether the pass of said lens means across said record medium is aligned with a track of information to be read from said medium; adjustment means responsive to said detection means for adjusting the position of said record medium when the pass of the lens means is not aligned with the track so that the next pass of said lens means across said record medium is substantially aligned with said track; and data recovery means receiving said optical beam after said beam impinges on said record medium and recovering said information from said medium. 2. The optical system as defined in Claim 1 wherein said detection means receives said optical beam after said beam has been reflected from said medium, said detection means detecting when said optical beam crosses a pair of tracks of information during said pass and the angle of rotation of said support means between EET the track crossings and calculating therefrom correction signals, said correction signals representing the required angle of rotation of said record medium to remove substantially all of the differences between the arcuate path traced by said lens means and said recorded track. 3. The optical system as defined in Claim 2 wherein said record medium is pivoted in response to said correction signals, said record medium being pivoted by the required angle so that a line extending along the longitudinal axis of said record medium passing through the centre of curvature of said track intersects the axis of movement of said support means. 4. The optical system as defined in Claim 3 wherein said adjustment means includes a carriage, said carriage being moveable along a first axis to allow successive tracks of information to be read from said medium and being moveable in a plane including said first axis in response to said correction signals to align the arcuate path traced by said lens means with said track of information to be read. 5. The optical system as defined in Claim 4 wherein said carriage includes an outer frame assembly and an inner frame assembly pivotally mounted at one end to said outer frame assembly, said outer frame assembly being in communication with a drive motor for advancing said outer frame along said first axis, said inner frame being in communication with an actuator, said actuator being responsive to said correction signals and pivoting said inner frame by said required angle to align the SUBSTITUTE SHEET track of information with said arcuate path in response to said correction signals. 6. The optical system as defined in Claim 5 wherein said system further includes a second light source mounted on said carriage and a light detector mounted near said second light source, said light detector being in communication with a closed loop position control circuit and said light source to allow the position of said inner assembly with respect to said outer assembly to be determined. 7. The optical system as defined in Claim 6 wherein said detection mean determines the required angle of rotation by taking the arc sine of the distance between the two crossed tracks divided by the distance between the two track crossings. 8. An optical recording and/or playback system comprising: a light source for generating an optical beam; support means for supporting at least one lens means; drive means for moving said support means to pass intermittently said lens means across a record medium; means for directing said optical beam through said lens means to impinge on said record medium; focus means receiving said optical beam prior to said optical beam impinging on said medium; detection means receiving said optical beam after said beam has impinged on said record medium and detecting whether said optical beam is focussed on said medium, said detection means generating focus error signals when said beam is not focussed on said medium; focus correction means operable in a first mode when said lens means is passing across said record medium and a second mode when said lens means is reote from said record medium, in said first mode, said focus correction means being responsive to said focus error signals and being operable to adjust said focus means to maintain the focus of said optical beam on said record medium and in said second mode, said focus correction means maintaining said focus means in a predetermined position. 9. The system as defined in Claim 8 further comprising sensing means in communication with said focus means, said sensing means monitoring the movement of said focus means and detecting the position thereof when said focus error signal is substantially equal to zero and wherein said predetermined position is chosen to be substantially identical to the position of said focus means in the previous pass of said lens means across said medium when said focus error signal was substantially equal to zero. 10. The system as defined in Claim 9 wherein said focus means and said lens means form a single objective lens assembly, said assembly being moveable in response to said focus correction means. 11. The system as defined in Claim 9 wherein said sensing means comprises a second light source and a light detector array, said second light source generating an optical beam towards said focus means, said focus menas being provided with a reflecting SUBSTITUTE SHEET surface and reflecting said beam towards said array, said array generating position signals indicating the position of said focus means; and register means receiving said position signals and storing said position signal when said focus means is passing across said record medium and said focus error signal is substantially equal to zero. 12. The system as defined in Claim 11 wherein said register means is a sample and hold circuit, said circuit being connected to said focus correction means via a switch, said switch being open in said first mode and closed in said second mode. 13. The system as defined in Claim 8 further comprising medium detecting means for detecting the distance of said lens means from said medium as said lens means approaches said medium, said focus correction means further comprising linear movement means operable in said second mode, said linear movement means moving said focus means from said predetermined position at a constant rate commencing at a predetermined distance from said medium; said detection means detecting when said optical beam attains focus on said medium and providing signals to said linear movement means so that the predetermined distance from said medium is adjusted for the next pass of said lens means across said medium therby allowing focus of said optical beam on said medium to be attained more quickly. 14. The system as defined in Claim 13 wherein said medium detecting means includes an encoder and a timing generator, said encoder and generator determining the position of said lens means with respect to said medium, said movement means including a ramp generator connected to said focus correction means via a switch, said switch being open in said first mode and closed in said second mode. 15. An optical recording and/or playback system comprising: a light source for generating an optical beam; support means for supporting at least one lens means; drive means for moving said support means to pass intermittently, said lens means across a record medium; means for directing said optical beam through said lens means to impinge on said record medium; focus means receiving said optical beam prior to said beam impinging on said medium; detection means receiving said optical beam after said beam has impinged on said record medium and detecting whether said optical beam is focussed on said medium, said detection means generating focus error signals when said beam is not focussed on said record medium; and focus correction means operable in a first mode when said lens means is passing across said record medium and asecond mode when said lens means is remote from said record medium, in said first mode, said correction means being responsive to focus error signals and being operable to adjust said focus means to maintain the focus of said beam on said record medium and in said second mode, said correction means detecting the distance of said lens means from said medium as said lens means approaches said medium, and moving said focus TE SHEET means from a predetermined position to another position at a constant rate commencing at a predetermined distance from said medium; said detection means further detecting when said optical beam attains focus on said record after passing over data recorded on said record medium and providing signals to said focus correction means so that the predetermined distance from said medium is adjusted for a successive pass of said lens means across said medium so that focus of said optical beam on said medium is attained more quickly during said successive pass of said lens means across said record medium so that focus is obtaibned prior to said optical beam passing over data recorded on said medium. 16. The system as defined in Claim 15 wherein said focus correction means includes an encoder and a timing generator, said encoder and generator determining the position of said lens means with respect to said medium, said correction means including a ramp generator operable to move said focus means at said constant rate upon actuation of a switch, said switch being open in said first mode and closed in said second mode. 17. A method of determining the angle of rotation of a record medium to remove eccentricity occurring between an optical beam intermittently traversing arcuate paths across a record medium and spaced longitudently arcuate tracks recorded on said record medium, said method comprising: determining whether said optical beam crosses a pair of successive tracks; measuring the distance between said tracks and TE SHEET the distance of the arc traversed by the beam between said track crossings; and taking the arc sine of said distance between said tracks divided by the distance between said track crossings. 18. The method of Claim 17 wherein said track crossings are detected by examining tracking error signal magnitudes. 19. The method of Claim 16 further comprising the step of: rotating said record medium by said angle to align the arc traversed by said optical beam over said record medium with said track of information. 20. An optical recording and/or playback system comprising: a light source for generating an optical read beam in a read mode and an optical write beam in a record mode; focussing and directing means for receiving and directing said optical beam along a path on a record medium, said record medium having at least one track of information recorded thereon; detection mean3 receiving said optical beam after said beam has impinged on said record medium in said read mode and determining the beginning and ending positions of said track; processing means communicating with said detection means, said processing means interpolating between said beginning and ending positions using a typical path followed by said beam across said record medium and generating therefrom profiling signals; and BSTITUTE SHEET correction means for altering the position of said optical beam on said medium, said correction means being responsive to said profiling signals for altering the path of said optical beam in said write mode so that a track of information to be recorded thereon represents an attempt to replicate the shape of the previously recorded track. 21. The system as defined in Claim 20 wherein said correction means comprises tracking and focussing means, said tracking and focussing means monitoring said detection means and generating therefrom tracking and focus error signals in the presence of a recorded track, said processing means receiving said tracking error signals and generating therefrom said profiling signals. 22. The system as defined in Claim 21 wherein said processing means includes sampling means and a microprocessor based circuit responsive to timing signals, said sampling means sampling said tracking error signals at said beginning and ending positions, said microprocessor based circuit processing said sampled signals to form said profiling signals. 23. An optical data recording and playback system operable in a record mode and in a playback mode comprising: a light source for generating an optical write beam in said record mode and an optical read beam in said playback mode; focussing and directing means receiving said optical beam in said record and playback modes and directing said optical beam along a path on a record medium, said record medium having at least one track of information recorded thereon; correction means operable in a first mode in said record mode, in said first mode said correction means being responsive to profiling signals for altering the path of said write beam on said medium such that each successive track of information to be recorded thereon represents an attempt to replicate the shape of the previously written track; said correction means in said playback mode being responsive to tracking and focus error signals for altering the position of said read beam on said medium such that said optical beam follows the track of information to be read; light detection means for receiving said optical beam from said medium to detect said recorded information in said playback mode, said detection means detecting the beginning and ending of a recorded track of data on said medium; and processing means communicating with said light detection means, said processing means interpolating between said beginning and ending positions and generating said profiling signals. 24. The system as defined in Claim 23 wherein said correction means comprises tracking and focussing means, said tracking and focussing for monitoring said detection means and generating therefrom said tracking and focus error signals in the presence of a recorded track of data, said processing means receiving said tracking error signals in said record mode and generating therefrom said profiling signals, said correction means being responsive to said tracking and focus error signals in said playback mode to maintain said optical beam on said track. 25. A method of maintaining focus of an optical beam on a record medium in an optical system, said system including support means operable to pass intermittently a lens means across said medium, light detection means for detecting whether said beam is focussed on said medium and focus means operable to maintain the focus of said beam on said medium in response to focus error signals generated by said detection means, said method comprising the steps of: adjusting said focus means in response to focus error signals whilst said lens means is moving across said medium to maintain the focus of said beam on said medium; and locking said focus means in a predetermined position whilst said lens means is removed from said medium. 26. The method of Claim 25 wherein said predetermined position is determined to be the position of the focus means during the previous pass of said lens means across said medium when said focus error signal was substantially equal to zero. 27. The method of Claim 25 further comprising the step of moving said focus means at a constant rate from said predetermined position towards a second position while said lens means is removed from said medium at a predetermined distance from said medium as said lens means approaches said medium, said focus means being moved at a rate such that the best focus position of said focus means is achieved after said lens means reaches said medium. 28. A method of maintaining focus of an optical beam on a record medium in an optical system, said system including support means operable to pass intermittently a lens means across said medium, light detection means for detecting whether said beam is focussed on said medium and focus means operable to maintain focus of said beam on said medium in response to focus error signals generated by said detection means, said method comprising the steps of: adjusting said focus means in response to focus error signals whilst said lens means is moving across said medium to maintain the focus of said optical beam on said medium; and moving said focus means at a constant rate from a predetermined position towards a second position while said lens means is removed from said medium, at a predetermined distance from said medium as said lens means approaches said medium, said focus means being moved at a rate such that the best focus position of said focus means is achieved after said lens means reaches said medium. 29. The method of Claim 28 wherein said predetermined distance is adjusted during successive passes of said lens means over said medium to reduce the time taken for said focus means to reach the best focus position after said lens means passes the leading edge of the medium. 30. A method of maintaining an optical beam on a track of data recorded on a record medium in an optical system, said system including support means operable to pass intermittently a lens means across said medium, detection means for detecting whether said beam is on said track of data and tracking means operable to maintain said beam on said track of data in response to tracking error signals, said method comprising: adjusting said tracking means in response to tracking error signals whilst said lens means is moving across said medium to attempt to maintain said beam on said track of data; moving said tracking means to a default position when said lens means is removed from said medium; and damping said tracking means whilst said lens means is removed from said medium to inhibit oscillation thereof. 31. The method of Claim 30 further comprising the step of: monitoring the difference in magnitude in the tracking error signals between successive predetermined intervals along the pass of said lens means across said medium; and applying the difference between said signals to said tracking means to adjust the position thereof. 32. A method of recording a track of data on a record medium having at least-one track of data recorded thereon comprising the steps of: examining the starting and finishing points of the track of data recorded on said medium; interpolating between said points to define an optical beam path; and moving an optical beam along said path across said medium to record another track of data thereon, said other track of data having a shape substantially identical to said one track of data.

Inventors: ADAMSON G JOHN, CHU CHING, DRAGANOFF GEORGI, ADAMSON, G., JOHN, CHU, CHING, DRAGANOFF, GEORGI
Applicants: OPTICAL RECORDING CORP, OPTICAL RECORDING CORPORATION
Application year: 1990
Main classification: G11B7
IPC codes: G11B7