Abstract: A subaperture optical system for preventing interference between light inadvertently reflected from the protective overcoat of an optical recording disk from disruptively interfering with light reflected from the active layer of the optical recording disk, thereby causing coarse servo tracking errors as the coarse servo actuator carriage translates over the coarse servo tracks on the disk service. The optical system is comprised of a laser light source, a collimating lens system, an astigmatizing lens system, a subaperture mirror, a carriage actuator, a beam relaying telescope, an objective lens, an objective lens focus actuator, a reflected coarse servo beam focusing lens, and a coarse servo detector. The laser issues a beam which follows a first subaperture path that is parallel to, but off center from, the optical axis of an objective lens of the optical system. The beam is formed into a line focused spot which is focused on the disk surface at a non normal angle.

Abstract: An apparatus for recording pits and grooves on the surface of a recording medium such as an optical disk in which the pits and grooves are formed by separate light sources, thereby providing accurate control over the dimensions of both the pits and grooves. Light beams of separate wavelengths suitable for forming the pits and grooves are passed through respective optical modulators and continuously variable beam expanders, and then combined and applied through a focusing lens to the recording medium. A beam monitor and a focus monitor receive part of the light reflected from the surface of the disk to control the focus and beam size conditions.

Abstract: A low-cost recording disc rotary drive control apparatus having a simple circuit configuration employing no voltage divider or frequency-to-voltage converter. A position pulse signal with a period proportional to a position detecting signal produced by a position detecting device is generated, and a rotational pulse signal having a frequency proportional to the rate of rotation of the motor is generated. The supply of a control signal to the motor is effected while supply of an accelerating signal thereto is suspended using a switching device.

Abstract: A reproducing rate control apparatus for an information recording disc in which pieces of information and absolute times for respective information pieces (representative of the reproduction time measured from the innermost circumference of the information recording disc) are recorded on the disc at a constant linear velocity, a revolution number of the disc at a command position is predicted, rotation of a disc drive motor is controlled so that the disc rotates at a predicted revolution number, and an information reader adapted to read the information recorded on the disc is moved to the command position when the revolution number controlling is effected or after the revolution number controlling has been completed.

Abstract: An optical type disc is recorded with information signals on each sloping surface of a spiral or concentric V-shaped grooves formed on a recording surface thereof, wherein the information signals are recorded as rows of geometrical depressions or projections on each sloping surface of the V-shaped groove and the inclination angle of the bottom surface of the depression or the top surface of the projection is greater than the inclination angle of the sloping surface of the V-shaped groove. An optical type reproducing apparatus reproduces the signals from the disc by relatively scanning the sloping surface of the V-shaped groove by a reproducing spot having a diameter smaller than the width of the V-shaped groove.

Abstract: An improved method of optically writing and/or reading high density information involves: (1) predeterminedly positioning a flat, uniformly thick, flexible, optical disc with respect to an axis of rotation; (2) rotating the disc on such axis at a high speed and in a predeterminedly spaced relation to a predeterminedly shaped, smooth, featureless reference surface and (3) coupling a central region between the disc and reference surface to an ambient gas source. Related apparatus and disc configurations, including embodiments having flexible disc covers, also are disclosed.

Abstract: An optical disk drive apparatus, for data or information disks of which the data or information are able to record or playback on each of the recording tracks, comprises an optical rotary pick-up head actuator for both seeking and following to the recording track to read or write data signals by a light beam. The light beam is injected along the axis of rotation of the optical rotary pick-up head actuator; guided through a stick-shaped prism in cantilever form which is fixed by one side on the optical rotary pick-up head actuator and is unsuspended by the other side; and focused upon the recording surface of the disk through an objective lens which can be rotatively moved and controlled with the optical rotary pick-up head actuator in the direction to seeking the tracks. An reflective light beam from the recording surface of the disk is guided in the reverse order and is taken out from the center axis of rotation.

Abstract: An optical tape record/playback apparatus is provided for recording/reproducing information signals onto an optical tape. The record apparatus comprises laser sources which are modulated in accordance with the information signals. A sequence number is recorded by the laser sources to aid in the recovery of information. In the playback mode a light detector is arranged to intercept the light reflected from the optical tape. The reflected light contains the information which was recorded on the optical tape during the recording operations. The high packing density of the information recorded in the tape may affect the proper recovery of the recorded information. During the recording operation a sequence number is recorded in a special section of the optical tape to aid in the recovery of the recorded information signals.

Abstract: A method for storing and reproducing data by means of a standard compact-disk digital audio player. The bit groups in which the data words are contained are recorded at least three times, the analog output signal of the player being sampled and digitized during reproduction.

Abstract: An apparatus is described for reading optically coded discs. In this apparatus the photodiode signals are amplified by current amplifiers which are biased by the unipolar photodiode signal currents, so that offset-free class-B operation is obtained.

Abstract: A record/playback apparatus is provided for recording/reproducing information signals in cooperation with an optical tape medium. The record apparatus comprises a laser source which is modulated in accordance with the information signals. A switching mirror, which is interposed between the laser source and the optical tape, switches the light beam emitted by the source to alternately scan helical tracks across an optical tape. In the playback mode a light detector is arranged to intercept the light reflected from the optical tape. The reflected light contains the information which was recorded on the optical tape during the recording operation.

Abstract: An optical disc player has a carriage (10) which can be moved to project outside the housing (1) and carries a drive motor (2) having a spindle (4) with means (6) for supporting a disc (40). The player also includes an optical reading device (7) having an optical read head (8) which is movable over the surface of the disc. The reading device (8) is secured in the housing (1) and the housing comprises aligning means (11) which align the spatial position of the rotational axis (3) of the drive motor (2) accurately relative to the optical reading device (7).

Abstract: Disk cutting apparatus, in which a plurality of beams are generated simultaneously from a laser light source by means of an ultrasonic light deflector, of which beams enter a common focussing lens; the light deflector is so controlled that each spot of the plurality of beams is located on the surface of the disk exactly with a desired track pitch in the radial direction of the disk and when the disk is rotated once a plurality of coaxial tracks are formed simultaneously.

Abstract: A system for recovering data, both prerecorded and user data, from a reflective optical data storage medium, with the further capability of distinguishing data from foreign particulate matter on the data disk. A pair of detectors measures the fraction of light reflected from and transmitted through the data disk and, knowing the fraction of light absorbed by the disk, distinguishes user data from prerecorded data and particulate matter. The data disk has different optical characteristics for prerecorded and user data, the prerecorded information being black, highly absorptive silver, while the user data consists of pits of relatively high transmissivity in a reflective field. Particulate matter is recognized by intermediate reflectivity and transmissivity. The optical characteristics of the material are enhanced or exaggerated so that the two types of data and particulate matter can be distinguished.

Abstract: An optical disk apparatus has an optical head for recording information on or reproducing information from an optical disk. The optical disk has a number of spiral or concentric tracks and a number of sectors formed along the circumferential direction of the disk. The sectors are divided into a plurality of sector blocks along the circumferential direction of the disk. Each sector block includes a different number of sectors which are continuous along the circumferential direction and which have a substantially constant linear length. A memory stores at least the speed data for maintaining a constant relative speed between the optical head and optical disk and is determined for each block group having a predetermined number of blocks. Speed data corresponding to the designated sector block is read out from the memory and is supplied to a velocity control circuit. A motor is driven to maintain a constant linear velocity between the optical head and the tracks of the optical disk.

Abstract: An optical erasable disk memory system which utilizes duration modulated laser switching is disclosed. The disk storage system utilizes a disk-shaped storage medium which includes a planar substrate and a thin film of phase change material. A substantially transparent optical tuning layer and protective layer are also utilized in the preferred embodiment of the present invention. The phase change material utilized is sensitive to both stress and temperature variations and will change from a first optically discernible phase to a second optically discernible phase at any portion thereof subjected to a temperature in excess of a selected threshold temperature at a particular stress and will change from the second phase back to the first phase at any portion thereof subjected to a stress in excess of a selected threshold stress at a particular temperature.

Abstract: The device according to the invention realize the following steps:(a) recording of binary test words on an optical disc associated to distinct engraving optical powers and comprising monotonous series of "1" and "0" logic bits of equal duration;(b) reading of these words;(c) measurement of the durations of these bits in the "1" state and bits in the "0" state,(d) determination of the optical power for which this ratio is closest to one;(d) elaboration and putting in memory of a control signal allowing to obtain this power.In a preferred variant, several test words are recorded and an interpolation is carried out between the measurements.

Abstract: In a video and/or audio disc information recording and reproducing apparatus including a coarse tracking control means, to coarsely vary the positions of the turntable and the pickup means relative to each other in a radial direction of the disc on the turntable, is driven to move in a direction and over a distance to lessen the difference between the address picked up and the address to be searched and at a speed varying with the absolute value of the difference between such addresses and decreasing at a deceleration rate which increases as absolute value decreases within a predetermined range. The apparatus includes a fine tracking control means to minutely vary the relative positions of the turntable and the pickup means. The fine tracking control is made operative when the absolute value is reduced to the lower limit of the above mentioned predetermined range and the coarse tracking control means is made inoperative.

Abstract: An optical disk storage unit has an improved access and tracking system in which a low speed velocity signal is derived from a differentiated position error signal. The output of the differentiator is applied to a blanking circuit which disconnects the steep slope portion of the position error signal from a sample and hold circuit. A medium speed velocity signal is derived from the integral of motor current and a high speed velocity signal is derived by digitally counting the band crossings in the position error signal.

Abstract: A signal processor and method for frequency compensating the drive signal for an optical modulator in an optical recording apparatus to make the exposure level modulation amplitude on the medium independent of the spatial frequency of the information being recorded. The processor compensates for exposure level modulation roll off at high spatial frequencies due to the finite size of the recording spot by increasing the amplitude of the modulator drive signal at high frequencies. The compensation is preferably the inverse of the modulation transfer function of the recording objective lens.

Abstract: In an information storage system including a rotating recording medium having a number of information tracks, a head positioning mechanism for switching access position between adjacent tracks and a plurality of buffer memories each capable of storing one track of data, an alternate record area for storing data in substitution for an error data block is provided at an end of each of the tracks of the recording medium, and if an error data block is detected by a read-after-write check of the data during the rotation of the disc after the data has been written in one of the tracks from one of the buffer memories during the previous rotation of the disc, the data from the other buffer memory is written in the adjacent track or the data is read from the adjacent track and checked during a disc rotation waiting time required before the data write operation to the alternate record area is started.

Abstract: Apparatus and method for monitoring the optical recording of information on an information storage member using a first beam of light for writing the information and a second beam of light for reading the information. Both the writing beam and the reading beam are focused on the storage member by a single objective lens. The input information signal which is used for modulating the writing beam is also directed to a delaying circuit, and the delayed information signal is compared with the playback signal derived from the reading beam.

Abstract: A method of moving a pick-up device of an optical reproducing apparatus to a position corresponding to a desired address of a record disk, the record disk having an information signal and an address signal recorded in an information area thereon, comprising the steps of producing a desired address signal corresponding to the desired address; partitioning the information area into radially partitioned information sections; moving the pick-up device to a position between two adjacent radially partitioned information sections at a speed faster than normal reproducing speed; reproducing the address signal at the last position of the pick-up device; comparing the reproduced address signal at the last position with the desired address signal; further moving the pick-up device in a direction and an amount determined by the comparing step to a position between two adjacent radially partitioned information sections and within one of the radially partitioned information sections between which the pick-up device was las

Abstract: The optical head in the recording - reading device of a data carrier comprises a semiconductor laser source. A lens makes it possible to focus the beam from this source on to the data carrier via a reflecting or deviating mirror. According to the invention the rotation axis of this mirror is in the plane of the entrance pupil and of the focus of the focusing lens.

Abstract: The rotation speed of a disc rotation motor is controlled such that a synchronizing signal derived from an output signal reproduced from a disc coincides with a reference oscillation frequency. The amount of data accumulated in a memory provided for absorbing jitter of the output signal reproduced from the disc is detected. In response to a result of the detection, a control signal in the disc rotation control is corrected such that the amount of data accumulated in the memory becomes substantially constant.

Abstract: An apparatus and method are disclosed for piecemeal recording of data on an optical disc using a spiral track format without the necessity of leaving a guard band between recordings or even providing a leader on the subsequently recorded track. A new recording is added to the end of a previously recorded track by first playing back the previous track. The tracking servo is locked to the data track and the translation stage motion is adjusted to match the spiral rate of the track. As the end of the previous track is approached, the translation stage drive is switched to a constant dc drive, the tracking servo is disabled, the tracking mirror is electronically locked to a nominal position, and the new recording begins. This operation may be repeated for each new data file to be recorded in piecemeal fashion.

Abstract: Method and apparatus for optically recording information on a moving information storage member and for reading the information immediately after it is written. An intensity modulated writing beam and a separate reading beam of light are simultaneously focused by a single objective lens. The reading beam impinges the medium at a point trailing the point of impingement of the writing beam by a prescribed amount. The reading beam is reflected by the moving medium in accordance with the recorded information. The modulated reflected light is collected by the objective lens, and the intensity of the reflected light is sensed for generating a playback signal. The quality of the playback signal may be monitored to allow control of the intensity of the writing beam.

Abstract: An optical storage medium incorporates stacked servo and data layers with the pattern of the servo layer being prepared prior to preparation of the data layer. The servo pattern comprises an undulation in a very thin film of high optical index material on the surface of the substrate of the medium. The undulating layer containing the servo pattern is separated from the active layer of the medium by a thin dielectric spacer film. The data layer is employed for formation of vesicular, or ablative or other marks forming surface discontinuities formed by any of the usual pit forming techniques. The preformatted servo information formed in the servo layer as buried undulations in the thin film is read out in amplitude or phase contrast. The thin undulating layer is of a significantly different index of refraction from the substrate and the next intervening layer. This layer can be transparent or semi-transparent.

Abstract: A system and method for maintaining a prescribed track pitch in an optical storage system wherein a second radiation beam is aligned a desired distance from a first radiation beam, the first radiation beam being controllably steered to follow a previously written data track on a record carrier used within the optical storage system. A write alignment servo aligns the second radiation beam the prescribed distance from the first radiation beam in response to an offset signal. The offset signal is generated by optically measuring the distance between a pair of calibration tracks that have previously been placed on the disk so as to have the desired track pitch therebetween. The effect of variations or drift occurring in the optical or electrical components of either the write alignment servo or the optical track pitch measurement system is significantly reduced by regularly calibrating against the fixed track pitch of the calibration tracks.

Abstract: A system and method for optically measuring and maintaining track pitch in an optical storage system. A set of at least four calibration tracks, each having a prescribed track pitch or distance therebetween, are precisely placed on a record carrier of the optical storage system during the manufacture thereof. When this record carrier is subsequently inserted into the optical storage system, the track pitch of a select pair of the calibration tracks is optically measured, and an offset signal is generated in response to this measurement. This offset signal is used within the optical storage system for controllably positioning a second radiation beam with respect to a first radiation beam, the first radiation beam being directed to follow a previously written data track on the record carrier.

Abstract: A system and method for optically measuring and maintaining track pitch in an optical storage system. A set of at least four calibration tracks, each having a prescribed track pitch or distance therebetween, are precisely placed on a recording medium of the optical storage system during the manufacture thereof. When this recording medium is subsequently inserted into the optical storage system, the track pitch of a select pair of the calibration tracks is optically measured by maximizing the amplitude of a data read radiation beam, and an offset signal is generated in response to this measurement. This offset signal is used within the optical storage system for controllably positioning a second radiation beam with respect to a first radiation beam, the first radiation beam being directed to follow a previously written data track on the recording medium.

Abstract: An optical head is provided for writing and reading a disk by means of at least one focused beam, the disk being provided with at least one track having uniformly spaced grooves. The optical head makes use of a motor comprising a fixed magnetic circuit provided with at least one air-gap in which a moving system is capable of displacement and is also provided with an access and radial tracking magnetic circuit including at least one core which is secant with all the grooves. The moving system is constituted by two actuators, the first actuator being intended to permit access and radial tracking while the second actuator permits focusing of the light beam on the track, the two actuators being coupled by flexible attachment means. Guiding means comprising magnetic elements permit lifting of the access and radial tracking actuator with respect to the core of its magnetic circuit.

Abstract: An optical disk recording and reproducing system which uses an optical disk at least one side of which is formed with a spiral groove which has a V-shaped or inverted-trapezoid-shaped cross sectional configuration. Both the radially inward and outward sloped faces of the groove are used as recording tracks which are tracked or scanned with the spot of a light beam in the case of recording or reproduction.

Abstract: An optical and reversible recording and reproducing apparatus providing a practical apparatus in which information signals can be recorded, reproduced and erased by irradiating a laser beam and the like on the substrate to be optically recorded. More practically, there is provided such an apparatus wherein two light sources such as lasers having different wavelengths are used, and a first almost circular micro-lightspot is produced by one light source, while a second elliptic light spot having a longitudinal diameter in the tangential direction of a guide track on a disc is produced by the other light source. These first and second light spots are closely irradiated on one and the same guide track of an optical recording disc. The recording and reproducing of the signals are performed using the first light spot while the erasure of the recorded signals can be made by using the second light spot or using both first and second light spots in combination.

Abstract: An optical recording and reproducing head wherein light beams emitted from two light sources are combined and applied to a recording medium and the light beams reflected from the recording medium are drawn out in such a way that they are separated from each other. The wavelengths of the light beams from both light sources are different from each other, i.e., .lambda..sub.1, .lambda..sub.2, and the light beams from the light sources are combined by means of a polarization beam splitter, two quarter wave plates and optical filters, and are then simultaneously focussed by a single focussing lens substantially along the optical axis thereof, thereby allowing microspots of focussed light to be disposed on a disc close to each other. The light beams reflected from the disc are drawn out so as to be completely separated from each other by the polarization beam splitter owing to the difference in polarization direction.

Abstract: A tracking servo system of a recording-disc reading and reproducing apparatus in which the information carried on a recording disc adapted to be driven for rotation is scanned by a signal pickup point displaced radially of the disc, comprising tracking error signal generator operative to detect an amount of deviation, if any, of the signal pickup medium from a target track on an information-carrying surface of the recording disc and produce a tracking error signal representative of the detected amount of deviation, and band eliminator such as a notch filter operative to eliminate a predetermined frequency in the tracking error signal.

Abstract: Apparatus for recording data by means of a beam of radiation on a recording medium which upon exposure to the beam undergoes optically detectable changes in the form of pits, which apparatus includes means for reducing the beam intensity as a result of pit initiation thereby forming more accurate pits.

Abstract: There is disclosed method and apparatus for recording and retrieving video information in two modes on a single laser recording disc. An annular portion of the disc carries still frame information recorded at a constant angular velocity. One still frame occupies a track or groove extending around exactly one full revolution of the disc recorded at constant angular velocity. Thus, any given still frame is presented on the video screen by repeatedly scanning one track for so long as that particular still frame is desired to be shown on the video screen. Another annular portion of the disc carries video information, i.e. the dynamic parts of a video program, recorded at constant linear velocity along the track or groove being scanned on the disc. A scanning head may be selectively shifted beween the two annular regions in order to incorporate predetermined still frames into the video program at desired spots during the program.

Abstract: In order to eliminate crosstalk and to provide a high recording density, a method for recording information on a recording disc includes a first step for forming a first portion of recording tracks in which the information signal is recorded on the recording disc under a condition that the angular velocity of rotation of the disc is constant and a space between each track has a first constant value. A second step in values forming a second portion of recording tracks in which the information signal is recorded on the recording disc under a condition that the angular velocity of rotation of the recording disc is varying and the space between each track has a second constant value that is greater than the first constant value. The first and the second portions of the recording tracks are formed alternately.

Abstract: An apparatus reads and/or records information on a disc by means of a light beam, which disc rotates about an axis of rotation, and is driven by a drive spindle which is journalled in a spindle support, an objective 1 being moved along two parallel guides by means of a slide in a radial direction relative to the axis of rotation. All parts of an opto-electronic system for producing the light beam and for effecting a conversion between optical and electrical modulation may be arranged on the slide but may alternatively form part of a stationary opto-electronic system. At least the spindle support or the stationary opto-electronic system is supported by at least one of the guides, for example by means of supports and which are formed with locating stops which cooperate with the guides 9A and 9B.

Abstract: In an optical disc having a file region and a directory region for carrying various pieces of information or data, one or more designated information can be erased by irradiating a laser light beam on a corresponding region in the file region where the light beam is modulated by a superposing writing signal which may be a d.c. or a predetermined a.c. signal. When it is intended to renew the information, designated information is first read out and stored in an external memory prior to performing superposing writing. Meanwhile, the data is renewed and is then written on an unused region in the file region. The directory of the renewed data region is also written so that the renewed information will be read out by searching the new directory. In another embodiment of the invention, the information in the file region is not erased to renew the same, but its directory is erased by superposing writing.

Abstract: A frequency modulated (FM) signal is recorded as pits or dots along a track formed as a concentric or spiral shape on a recording medium such as a recording disc. The pits or dots are arranged on a radial line during at least a predetermined interval.

Abstract: The invention relates to an apparatus for reading a disc-shaped record carrier with a spiral information track in which an optically readable digitally coded information signal is recorded with a spatial bit frequency which is independent of the radius of the track turn. The scanning speed of the information track is selected to be a factor n higher than the scanning speed normally employed of the reproduction for the recorded information. By means of a memory device, information blocks being read and comprising a fixed number of data bits are time-expanded to restore the customary time relationship. A control unit determines the read-in cycles of the memory device for the storage of the information blocks being read and, via the radial positioning system for the read spot, it determines the scanning pattern of the record carrier so that the ultimately recorded information blocks are reproduced in the same sequence and at the desired rate.

Abstract: An optical disc assembly includes an optical disc on which signals are recorded, and from which the signals are optically reproduced; a closed casing rotatably containing the optical disc, and having a transparent portion; and a rotor fixed at the center of the optical disc and magnetically coupleable with an external rotating magnetic field to be rotated therewith. An optical disc assembly includes a rotating magnetic field generating device arranged outside of the closed casing and concentrically with the optical disc to drive the latter without contact.

Abstract: An apparatus for optically reading a recording medium having information tracks disposed on alternating inclined recording surfaces preferably arranged in separate but alternately interposed spirals inclined at different angles on the medium. The apparatus includes photodetectors for detecting light reflected from each of the tracks, and circuitry for controllably combining the signals from the photodetectors to minimize crosstalk in the recovered signals.

Abstract: A disk recording device of the invention has a detector for detecting the position of an optical head with respect to the radial direction of a disk, using an optical scale, and a position signal processing circuit for generating a position signal from the output signal from the detector. An output signal from the circuit is converted by a D/A converter into a voltage signal, which is supplied to a voltage controlled oscillator. The oscillator generates a frequency signal. The frequency signal is supplied to a page buffer storing video data and one input of an AND circuit. The other input of the AND circuit is connected to receive a video data from the page buffer. The AND circuit generates a logic product of the frequency signal and the video data, or a recording timing signal having a frequency which is proportional to the position of the head with respect to the radial direction of the disk.

Abstract: The quality of spot-focus of optical writing and reading light upon the information surface of a flexible optical disc is passively enhanced by (1) rotating the disc with its neutral plane stabilized in space and (2) directing such light through the optical disc support into precise spot-focus on the information surface. With this technique spherical aberration induced by the disc support compensates for thickness variation of the support. Various structural embodiments are disclosed.

Abstract: An optical disk device which has a memory circuit for storing an eccentric displacement for one revolution of an optical disk. The memory circuit receives a signal from an eccentric displacement detector which detects the eccentric displacement of the optical disk on the basis of an optical signal from an optical head. In the recording and reproducing modes, the position of the optical head is controlled to compensate for the eccentric displacement on the basis of the eccentric displacement data stored in the memory circuit.

Abstract: An optical recording device for forming recording tracks of an indented pattern on a magnetic recording tape comprises a rotary drum which has at least two windows, the magnetic recording tape being driven obliquely on the windows along an outer surface of the drum, and an optical system, disposed inside the rotary drum, for converting a modulated laser beam incident on the optical system into two slit beams, major axes of sections of which form a predetermined angle and for emitting the slit beams on the magnetic recording tape through the windows, whereby the recording tracks of the indented pattern in correspondence with the signal are formed obliquely on the magnetic recording tape with respect to a longitudinal direction of the magnetic recording tape by the slit beams so as to form different azimuths between adjacent recording tracks.

Abstract: Method and apparatus for recovering information at a substantially constant rate from a rotatable information storage disc. The information is stored on the disc in a plurality of substantially circular and concentrically arranged information tracks, with a substantially uniform recording density, and it is recovered by controllably rotating the disc, relative to a transducer, at an angular velocity substantially inversely proportional to the radius of the corresponding track. The apparatus includes a coarse speed control potentiometer for producing a measure of the radius of the particular information track from which information is being recovered, and fine speed control means for comparing the relative phase angles of a periodic signal in the recovered information and a periodic reference signal and for producing a fine speed control signal proportional to the phase difference.