GB2046480A - Tracking servo and playback system for video disc - Google Patents

Tracking servo and playback system for video disc Download PDF

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Publication number
GB2046480A
GB2046480A GB8009833A GB8009833A GB2046480A GB 2046480 A GB2046480 A GB 2046480A GB 8009833 A GB8009833 A GB 8009833A GB 8009833 A GB8009833 A GB 8009833A GB 2046480 A GB2046480 A GB 2046480A
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United Kingdom
Prior art keywords
stylus
track
signal
electrodes
information
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
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GB8009833A
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Toshiba Corp
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Toshiba Corp
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Publication date
Priority to JP3612279A priority Critical patent/JPS55129945A/en
Priority to JP3612879A priority patent/JPS55129948A/en
Application filed by Toshiba Corp filed Critical Toshiba Corp
Publication of GB2046480A publication Critical patent/GB2046480A/en
Withdrawn legal-status Critical Current

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    • GPHYSICS
    • G11INFORMATION STORAGE
    • G11BINFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B3/00Recording by mechanical cutting, deforming or pressing, e.g. of grooves or pits; Reproducing by mechanical sensing; Record carriers therefor
    • G11B3/44Styli, e.g. sapphire, diamond
    • G11B3/46Constructions or forms Disposition or mounting, e.g. attachment of point to shank
    • GPHYSICS
    • G11INFORMATION STORAGE
    • G11BINFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B9/00Recording or reproducing using a method not covered by one of the main groups G11B3/00 - G11B7/00; Record carriers therefor
    • G11B9/06Recording or reproducing using a method not covered by one of the main groups G11B3/00 - G11B7/00; Record carriers therefor using record carriers having variable electrical capacitance; Record carriers therefor

Abstract

A tracking servo and playback system for a video disc has an information track on which information corresponding to video and audio signals is recorded, a stylus to trace the information track is provided with a plurality of electrodes (21a, 21b) that are connected to their corresponding signal detector circuits (22a, 22b). The tracking servo system corrects the tracking position of the stylus. For this purpose, the output of one signal detector circuit and a reference voltage are applied to a comparator (25) the output of which is used to drive tracking compensation means (27a, 27b). A demodulator (29) is connected to the output of at least one signal detector circuit. The circuit shown is for a stylus having two equal electrodes, each covering half of a track; in one alternative only one such electrode is used for control, its output being compared with a reference voltage while a second electrode provides the information signal only. In other alternatives there are two electrodes for control only, the information signal being obtained from a third electrode. Physical arrangements for the stylus suspension are described (Figures 11, 12). <IMAGE>

Description

SPECIFICATION Tracking servo and playback system for video disc This invention relates to a playback and tracking servo system for video discs.

There are generally known video playback systems which reproduce FM signals including video and audio signal components from a rotating video disc which has a spiral track provided with pits corresponding to the FM signals by bringing a stylus into contact with the track to detect changes in capacitance between an electrode formed on a plane of the stylus and an electrode formed on the video disc which are caused by existence of the pits. The video and audio signal components are extracted from the reproduced FM signals by the frequency separation method, and are separately subjected to additional demodulation.

There are grooved and ungrooved type video discs. In a grooved type video disc, a groove is formed along a track, and pits are defined at the bottom of the groove. In the case of such grooved type disc, therefore, the stylus traces the groove, so that there is no need of tracking servo. Since the stylus traces the groove, however, playback of the video disc is limited to the normal playback mode; it is difficult to achieve picture reproduction in special playback modes, such as quick-motion mode, slowmotion mode, still-picture mode, etc.

In order to enable picture reproduction in the special playback modes, there has been developed such an ungrooved type disc as shown in Figure 1. In Figure 1, numeral 11 designates a disc base formed of synthetic resin on which pits 12 are formed along a recording track. Signal information is recorded as changes of the length of the pits along the track and the space between the pits. A metal coating layer 13 is formed on the disc base 11 with the pits thereon, and a dielectric layer 14 formed on the metal coating layer 13. The dielectric layer 14 is coated with a smoothing agent 15 so that the surface of the video disc may be smooth or even. Numeral 16 denotes a sapphire or diamond stylus. An electrode 17 is formed by evaporation on a surface of the stylus 16 parallel to the traverse direction of the recording track.The capacitance between the electrode 17 on the stylus 16 and the electrode 13 of the disc base changes according to the recording information as the video disc rotates.

In such ungrooved video disc, although special picture reproduction may be easily performed due to evenness of the surface of the disc, a tracking servo system is essential to accurate tracing of the recording track. Such a video disc as shown in Figure 2 has been developed fo tracking servo performance. In this video disc, tracking information pits (Cm~1, Cm, Cm+1; Con~1, On, Con+1) are defined between adjacent information pits arranged along a recording track TR. The groups of tracking information pits (Cm~1, Cm, Cm+1) and (On-i, Cn, Con+1) are recorded at different frequencies.The tracking information, along with video and audio signal information, is picked up by the reproducing stylus 16, and is frequency-separated into tracking information signals corresponding to the pits (On-i, On, Con+1 ) and tracking information signals corresponding to the pits (0m-i, Cm, Cm+1 ). While the stylus is correctly tracing the recording track, the quantity of tracking information corresponding to the pits (Cn-1, Cn, Con+1) and the quantity of tracking information corresponding to the pits (Cm-i, Cm, Cm+1 ) are equal.

The quantity of tracking information may be obtained by rectifying the frequency-separated tracking information signals. The two quantities of tracking information are compared by means of a differential amplifier for tracking servo. If the stylus is biased toward the group of pits (On-i, On, Con+1) or (Cm~1, Cm, Cm+1 ), two input signals for the differential amplifier will differ from each other in magnitude, and a tracking compensating device responsitve to outputs of the differential amplifier will control the stylus so that it may correctly trace the recording track.Such tracking servo system would complicate the manufacture of video discs, and require sophisticated circuits for the detection of tracking information.

An object of this invention is to provide a tracking servo and playback system for video discs capable of stylus tracking servo operation without provision of any tracking information pits or guide groove on the disc.

According to this invention, a plurality of electrodes connected to their corresponding signal detector circuits are formed on a stylus to trace an information track, and the amplitude level of information signals demodulated by at least one signal detector circuit is detected to correct the tracing position of the stylus. The amplitude level of information signals detected varies with tracing position of the stylus.

According to a first embodiment of the invention, two electrodes which each cover a half width of the information track when the stylus is in its correct tracing position are formed on the same surface of the stylus. Two voltage signals corresponding to the amplitude levels of demodulated information signals from two signal detector circuits connected respectively to these two electrodes are compared by a comparator circuit, and a tracking servo device corrects the tracing position of the stylus in response to the comparator circuit. The demodulated signals from the two signal detector circuits are added together, and transmitted to a video and audio demodulator circuit.

According to a second embodiment of the invention, a first electrode for information signal detection and a second electrode for tracking servo are formed on different surfaces of the stylus. The first electrode covers the overall width of the information track, while the second electrode covers half the width of the information track. The demodulated signal from the signal detector circuit connected to the first electrode is supplied to a video and audio demodulator circuit, and the amplitude level of the demodulated signal from the signal detector circuit connected to the second electrode is compared with a reference voltage by a comparator circuit.

According to a third embodiment, a first electrode for information detection covering the overall width of the information track is formed on a first surface of the stylus, while second and third electrodes each covering a half width of the track portion traced by the first electrode are formed on a second surface of the stylus. The amplitude levels of the output signals of signal detector circuits connected respectively to the second and third electodes are compared for tracking servo by a comparator circuit.

According to a fourth embodiment, first, second and third electrodes are formed on one surface of the stylus so that the first electrode covers the overall width of the track and that the second and third electrodes cover respective halves of track portions on both sides of a track portion traced by the first electrode. The output signal of a signal detector circuit connected to the first electrode is supplied to a video and audio demodulator circuit, and the amplitude levels of the output signals of signal detector circuits connected respectively to the second and third electrodes are compared for tracking servo by a comparator circuit.

This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figure 1 is a sectional view of a prior art video disc with flat surface to which this invention is applied; Figure 2 schematically shows part of a prior art video disc on which tracking information is formed; Figure 3 shows the structure of a stylus according to a first embodiment of this invention; Figure 4 is a schematic block diagram for tracking servo and playback applied to the stylus structure of Figure 3.

Figure 5a is a circuit diagram showing an example of a signal detector circuit of Figure 4, and Figures 5b to 5d are wave forms occuring in the current of Figure 5a.

Figure 6 shows the structure of a stylus according to a second embodiment of the invention; Figure 7 is a schematic block diagram for tracking servo and playback applied to the stylus structure of Figure 6; Figures 8 and 9 show stylus structures according to third and fourth embodiments of the invention; Figure 10 is a schematic block diagram for tracking servo and playback applied to the stylus structures of Figures 8 and 9; and Figures 11 and 12 schematically show tracking servo systems which may be utilized for the invention.

Referring now to the drawing of Figure 3 showing a first embodiment of this invention, electrodes 21 a and 21 b of the same size are disposed on a first surface Sa of a sapphire or diamond stylus 20 with a pentagonal base in contact with the surface of a disc, corresponding to an information trackTR1 formed by a conventional method. Here the flat surface S1 is formed in parallel to the transverse direction (X-Y direction) of the track on the rotating direction side of the disc. The electrodes 21a and 21 b are so formed as to cover the halves of the in-formation track width on their corresponding sides when the stylus traces the track correctly, as shown in Figure 3.

Referring now to Figure 4, there will be described a signal playback and tracking servo system using the above-mentioned reproducing stylus. In Figure 4, numerals 21 a and 21 b designate the aforesaid electrodes on the stylus 20, which are connected to signal detector circuits 22a and 22b, respectively.

The signal detector circuits 22a and 22b reproduce information signals corresponding to the pits of the information track being traced in accordance with the aforementioned capacitance change. As shown in Figure 5, the signal detector circuit may be comprised of a coil 31 forming a tank circuit 30 together with a variable capacitance 32 between the stylus and disc electrodes, a UHF oscillator 33 with fixed oscillation frequency whose oscillation output is inductively coupled to the tank circuit 30, a coil 34 inductively coupled to the tank circuit 30, and a rectifier circuit 35 connected to the coil 34.

In the above-mentioned arrangement, oscillation frequency fl of the oscillator 33 is so set as to position on the slope of the resonance curve throughout the maximum changing range of the resonance frequency of the tank circuit 30, as shown in Figure 5(b). Figure 5(c) shows the input signal of the rectifier circuit 35, while Figure 5(d) shows the output signal of the rectifier circuit 35, that is, the FM information signal corresponding to the pits.

The output signals of the signal detector circuits 22a and 22b are amplified by amplifiers 23a and 23b, respectively. The output signals of the amplifiers 23a and 23b are coupled to a voltage comparator circuit or differential amplifier circuit 25 via rectifying circuits 24a and 24b having a relatively large time constant, respectively. The rectifying circuits 24a and 24b produce DO voltages corresponding to the amplitude levels of the signals reproduced by the signal detector circuits 22a and 22b, and supply them to the differential amplifier 25. Outputs of the differential amplifier 25 are coupled to tracking compensating coils 27a and 27b through tracking coil driver circuits 26a and 26b. As mentioned later, the tracking compensating coils 27a and 27b are disposed facing each other, and a cantilever bearing the stylus on its tip end extends between the coils.

Further, an iron foil is attached to the cantilever at a portion facing the tracking compensating coils 27a and 27b.

The output signals of the amplifiers 23a and 23b are supplied to a summing circuit 28, where they are added together. The output signal of the summing circuit 28 is supplied to a conventional video and audio demodulator circuit 29, where video and audio signals are demodulated separately.

In the operation of the above-mentioned system, as shown in Figure 3, the amplitude levels of the output signals of the signal detector circuits 22a and 22b are equal when the stylus 20 correctly traces the information track. Accordingly, the voltage levels of the outputs 25a and 25b of the comparator 25 are also equal, and the tracking compensating coils 27a and 27b which responds to the outputs of the comparator 25 have no effect on the cantilever with the stylus on its tip end.

Meanwhile, if the stylus 20 is biased to the left or right side of Figure 3, the width of contact between one of the electrodes 21a and 21b and the information track becomes larger than the contact width provided in correct tracing, while the width of contact between the other electrode and.the information track becomes smaller. Accordingly, the amplitude of the output signal of one of the signal detector circuits 22a and 22b is increased as compared with that in the case of correct tracing, while the amplitude of the output signal of the other is reduced. As a result, balance between the output levels of rectifying circuits 24a and 24b, i.e. between the levels of outputs 25a and 25b of the comparator 25, is lost.In response to this, one of the tracking compensating coils 27a and 27b is energized more strongly, while the other is energized more weakly, thereby restoring the cantilever to the correct tracing position. In Figure 4, the summing circuit 28 is intended substantially to eliminate variations in the level of the input signal to the video and audio demodulator circuit 29 which are caused by displacement of the stylus from the correct tracing position.

Figures 6 and 7 show another embodiment of the invention. In this embodiment, as shown in Figure 6, a stylus 20 has a hexagonal base, an electrode 40 as wide as the track width or pit width is formed on a surface S2 of the stylus opposite to the rotating direction of the disc, and an electrode 41 is formed on a surface S3 wider than the track width on the forward side of the disc rotation so as to cover one half of the pit width when the stylus traces the track correctly.

Figure 7 shows a signal processing circuit applied to the electrode structure of Figure 6. In Figure 7, like reference numerals refer to the same parts as shown in Figure 4. The stylus electrodes 41 and 40 are connected to signal detector circuits 22a and 22b, respectively. The output of the signal detector circuit 22a is coupled to one input of a comparator 25 through an amplifier 23a and a rectigying circuit 24a.

The other input of the comparator 25 is connected with a reference voltage source 42. The magnitude of the output voltage of the reference voltage source 42 is made equal to that of the output voltage of the rectifying circuit 24a which may be obtained when the stylus 20 traces the information track TR1 correctly as shown in Figure 6. Accordingly, when the stylus traces the information track TR1 correctly, tracking compensating coils 27a and 27b coupled to outputs 25a and 25b of the comparator 25 have no effect on the cantilever. If the stylus is biased, however, the output voltage of the rectifying circuit 24a will become higher or lower than the reference voltage. In response to this, the tracking compensating coils 27a and 27b correct the cantilever for position, causing the stylus to trace the information track TR1 correctly.Meanwhile, the output signal of the signal detector circuit 22b is applied through the amplifier 23b to a video and audio demodulator circuit 29, where it is subjected to necessary processing.

Figures 8 and 9 show stylus structures according to further embodiments of this invention. The stylus of Figure 8 has three electrodes 50,51 a and 51b which simultaneously trace the same pit train. That is, the stylus 20 has two surfaces S4 and S5 parallel to the transverse direction (X-Y direction) of the track, and the electrode 50 for information signal detection covering the overall width of the track is formed on the one surface S4, while the two electrodes 51a and 51b for tracking servo each covering one half of the track width are formed on the other surfaces S5. In the stylus structure of Figure 9, there are formed a signal detection electrode 50' and tracking servo electrodes 51'a and 51'b on a wide surface S6 of the stylus which covers three pit rows.The electrode 50' covers the overall width of each pit, while the electrodes 51 'a and 51 'b cover substantially half the widths of two pit rows on both sides of the pit row traced by the electrode 50', respectively.

As shown in Figure 10, a like signal processing circuit may be used with the stylus structures of Figures 8 and 9. That is, the tracking servo electrodes 51 a (51 'a) and 51 b (51'b) are connected to signal detector circuits 52a and 52b respectively, the output of the signal detector circuit 52a is coupled to one input of a voltage comparator 55 through an amplifier 53a and a rectifying circuit 54a, and the output of the signal detector circuit 52b is coupled to the other input of the comparator 55 through an amplifier 53b and a rectifying circuit 54b.One output 55a of the comparator 55 is coupled to a tracking compensating coil 57a through a tracking compensating coil driver circuit 56a, while the other output 55b of the comparator 55 is coupled to a tracking compensating coil 57b through a tracking compensating coil driver circuit 56b. The signal detection electrode 50 (50') is connected to a signal detector circuit 52c whose output is coupled to a video and audio demodulator circuit 58 through an amplifier 53c. The operation of the circuit of Figure 10 is excluded from further description because it may easily be understood from the operation of the aforementioned embodiment.

Referring now to Figure 11, there will be described an example of a tracking compensating device which can be applied to this invention. The abovementioned stylus 20 is attached to the tip end of the bottom side of a cantilever 20a obliquely penetrating an opening 61 in a base 60 of a pickup cartridge, and an iron piece 20b is mounted on the other end of the cantilever by means of a damper 20c. The iron piece 20b is supported by an elastic belt 63 attached to an upright wall 62. An iron foil 64 is fixed to the cantilever 20a at its portion intersecting the opening 61. A pair of tracking compensating coils 65a and 65b are disposed on the base 60, facing each other with the iron foil 64 therebetween. According to such construction, if the stylus is biased from its correct tracing position, the tracking compensating coils 65a and 65b are differently energized. As a result, the iron foil 64 is attractd toward the compensating coil 65a or 65b, so that the stylus is restored to the correct tracing position. The iron piece 20b is intended for jitter compensation, and a jitter compensating coil (not shown) is provided on the back side of the iron pice 20b. Numerals 66 and 67 designate elastic lead wires attached to the electrodes of the stylus 20 and connected to the signal detector circuits.

Figure 12 shows another example of a tracking compensating device. In this example, a permanent magnet 70 penetrates a bobbin 71 on which a pair of tracking compensating coils 72a and 72b are wound and which is supported by an elastic member 73. A member 74 with a slot 75 is disposed under the - bobbin 71, and the cantilever 20a passes through the slot 75. In this construction, the bobbin 71 moves along the magnet 70 in response to different currents flowing through the pair of compensating coils 72a and 72b when the stylus is not in its correct position, thereby biasing the cantilever to restore the stylus to the correct tracing position.

This invention may be applied to a video disc on which concentric tracks are formed, as well as to a video disc on which a spiral track is formed. Since the surface of the video disc is flat, the stylus can easily be transferred to an adjacent track in the vertical fly-back period.

Claims (9)

1. A playback system for a video disc having a spiral information track or concentric tracks on which a signal including video and audio information is recorded, comprising: a stylus having first and second electrodes formed at different portions thereof, said electrodes detecting the recorded information when said stylus traces the information track; first and second signal detector circuit means respectively connected to said first and second electrodes for demodulating the signal recorded on the information track; a comparator circuit having a first input coupled to said first signal detector circuit means and a second input connected to receive a comparison signal for comparing a voltage signal depending on an output signal of said first signal detector circuit with the comparison signal applied to the second input thereof to detect the tracing position of said stylus;; tracking compensating means coupled to outputs of said comparator circuit for restoring said stylus to the correct tracing position when said stylus is not in the correct tracing position; and demodulator circuit means coupled to said second signal detector circuit means for extracting video and audio information from an output signal of the second signal detector circuit means.
2. A playback system according to claim 1, wherein each of said first and second electrodes is so formed as to cover approximately half the width of the information track when said stylus traces the information track correctly, and wherein said second input of said comparator signal is coupled to the output of said second signal detector circuit means to receive said comparison signal.
3. A playback system according to claim 1, wherein the output signals of the first and second signal detector circuits are supplied to said demodulator circuit means through a summing circuit.
4. A playback system according to claim 2, wherein said first and second electrodes are formed on a surface of said stylus parallel to the transverse direction of the track.
5. A playback system according to claim 1, wherein said first and second electrodes are formed on two surfaces of said stylus which are parallel to the transverse direction of the track, so as to cover approximately half the width of the information track and the overall width of the information track, respectively, when said stylus is in the correct tracing position, and wherein said second input of said comparator circuit is connected with a reference voltage source to receive said comparison signal.
6. A playback system for a video disc having a spiral information track or concentric information tracks on which a signal including video and audio information is recorded, comprising: a stylus having first, second and third electrodes formed at different portions thereof, said electrodes detecting the recorded signal when said stylus trace the information track; first, second and third signal detector circuit means respectively connected to said first, second and third electrodes for reproducing the recorded signal; comparator circuit means having first and second inputs respectively coupled to said first and second signal detector means for comparing voltage signals depending on output signals of said first and second signal detector circuit means to detect the tracing position of said stylus;; tracking compensating means coupled to outputs of said comparator circuit for restoring said stylus to the correct tracing position when said stylus is not in the correct tracing position; and demodulator circuit means coupled to said third signal detector circuit means for extracting video and audio information from the output signal of said third signal detector circuit means.
7. A playback system according to claim 6, wherein said first and second electrodes are formed on a first surface of said stylus parallel to the transverse direction of the track so that each of said first and second electrode may cover approximately half the width of the track on each side when said stylus traces the track correctly, and said third electrode is formed on a second surface of said stylus parallel to the transverse direction of the track so as to cover the overall width of the track.
8. A playback system according to claim 6, wherein said first, second and third electrodes are formed on a plane of said stylus parallel to the transverse direction of the track so that said first and second electrodes are located respectively on both sides of said third electrode, that said third electrode covers the overall width of the track, and that said first and second electrodes each cover approximately half the width of a track portion on each side of a track portion traced by said third electrode when said stylus traces the track correctly.
9. Tracking servo and playback system for video disc, substantially as hereinbefore described with reference to the accompanying drawings.
GB8009833A 1979-03-27 1980-03-24 Tracking servo and playback system for video disc Withdrawn GB2046480A (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
JP3612279A JPS55129945A (en) 1979-03-27 1979-03-27 Disc reproducing device
JP3612879A JPS55129948A (en) 1979-03-27 1979-03-27 Disc reproducing device

Publications (1)

Publication Number Publication Date
GB2046480A true GB2046480A (en) 1980-11-12

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GB8009833A Withdrawn GB2046480A (en) 1979-03-27 1980-03-24 Tracking servo and playback system for video disc

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GB (1) GB2046480A (en)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0162349A2 (en) * 1984-05-24 1985-11-27 International Business Machines Corporation A data recording medium
US4639906A (en) * 1981-04-07 1987-01-27 Victor Company Of Japan, Limited Magnetic recording on tracking controlled grooveless medium having magnetic and capacitance properties
EP0450629A1 (en) * 1990-04-06 1991-10-09 Sony Corporation Recording and reproducing device and recording medium therefor

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
NL7301932A (en) * 1973-02-12 1974-08-14
FR2313716B1 (en) * 1975-06-03 1982-09-03 Thomson Brandt
JPS5535777B2 (en) * 1975-11-21 1980-09-17
US4331976A (en) * 1976-04-08 1982-05-25 Victor Company Of Japan, Ltd. High density recording system using side-by-side information and servo tracks

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4639906A (en) * 1981-04-07 1987-01-27 Victor Company Of Japan, Limited Magnetic recording on tracking controlled grooveless medium having magnetic and capacitance properties
EP0162349A2 (en) * 1984-05-24 1985-11-27 International Business Machines Corporation A data recording medium
EP0162349A3 (en) * 1984-05-24 1988-09-07 International Business Machines Corporation A data recording medium
EP0450629A1 (en) * 1990-04-06 1991-10-09 Sony Corporation Recording and reproducing device and recording medium therefor
US5488519A (en) * 1990-04-06 1996-01-30 Sony Corporation Recording and reproducing device having conductive spindle

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Publication number Publication date
DE3011909C2 (en) 1982-06-03
DE3011909A1 (en) 1980-10-02

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