Compact disc playback enhancer

Abstract: The playback of a compact disc is enhanced by adhering to the compact disc an annular ring to increase the frictional engagement between the disc and the spindle upon which it turns. The playback is further enhanced by adhering a layer of soft, flexible material to dampen vibrations caused by the variations in speed by which the compact disc turns.

Description: This invention relates to compact disc recordings, and specifically to a means for enhancing the reproduction of sounds from compact discs. Compact discs or CDs are recordings used in compact disc players having a laser with associated electronic circuitry to read information, such as recorded music, stored on the compact discs. The method used for converting the information stored on the compact disc is to rotate the disc on a spindle while the beam of a laser strikes the discrete bits of information on the compact disc and is reflected from the compact disc to a photosensor which reads the reflected signal and converts the light signal to a digital electronic signal. The electronic signal is conditioned and converted to an analog signal which is appropriately amplified and converted to sound energy through a loudspeaker. It was originally believed that the data stream recovered in a compact disc player was exceedingly robust, and that the sound quality was unaffected by microphony, transmitted vibration, and other influences. However, designers have since shown that a number of mechanical areas can and do affect player sound quality. Factors include the mass and decoupling of the compact disc transport itself, the dampening and mass of the disc clamps and of the support hub, plus the rigidity and the dampening of the case, for example, the top cover. Less vibration reaching the transport means less work for the laser head in its task of maintaining focus on the fast moving data track. In turn, better tracking means that fewer power drain variations are imposed on the overall system, with the probability of lower data error rates. In fact, even corrected errors can be shown to affect sound quality through power supply interactions. Thus, even though the compact disc has a very high theoretical capacity for high fidelity sound reproduction when sufficiently high effective sampling rates are employed, the musical quality of compact disc players has, nevertheless, been found to be limited. To some listeners, the resultant sound quality is described as harsh, grating, lacking in musical naturalness, piercing, booming, as having a notable loss of sound stage and presence and a general lack of naturalness. A wide variety of improvements have been proposed to cure these deficiencies, all of which generally are directed at some specific problem. A list of improvement features includes special sampling techniques, improved analog-to-digital converters, improvement in filters, special modulation codes, particular formatting, phase shift compensation, anti-aliasing filters, separation of laser and spindle mechanisms from signal electronics into differently housed components, improved audio cables of special construction, special turntable supports such as isolating platforms for minimizing the effects of vibration, and many more. The vibrations caused by audio feedback and stray light have been implicated in some tests regarding audio quality degradation. Proposed vibration cures include improved sub-chassis construction, special suspensions, and special compact disc player support tables which claim to help isolate vibration-caused defects. In theory, compact disc players have been said to possess immunity to such effects, but the proposed improvements all seem to make a suitable improvement in sound quality. Indeed, another cause of audio quality degradation may involve an intermodulation interaction between the vibration-induced fluctuations in the drive current requirements for the tracking servos which are working to remove such fluctuations as reflected into the digital circuitry and analog electronics through common connections such as power supplies. There is, therefore, a need for an improved compact disc playing system which will contribute to achieving a more natural, listenable, and satisfying compact disc reproduction, especially of musical material. One solution is proposed in U.S. Patent No. 4,726,007, issued to McCormack. This patent proposes the use of a rigid, reinforced damper disc made of material having significant loss to vibration and low sound propagation velocity. The damper disc is arranged to be laid over the compact disc and placed in loose physical contact with its upper surface to suppress and dissipate vibrational energy from the compact disc. The damper disc is light-opaque and light-absorptive. The quality of the sound produced by a compact disc player also requires that the data track on the compact disc be read at constant linear speed. It has become an audio standard to record and read compact discs along a spiral track at a constant linear speed by rotating the disc at variable speed. Because the track is arranged in a spiral configuration on the disc, the actual rotational speed of the compact disc is varied during the playback. Compact disc players thus operate at constant linear speed and variable angular speed. The compact disc player adjusts the angular or rotational speed of the disc during play so that the track from which information is extracted is passing through the laser beam at constant speed. In practice, the laser signal starts at the center of the compact disc and works its way out toward the outer edge. The imposition of constant linear speed results in the compact disc rotating faster at the start of play or at the center of the disc than at the end of play or at the outside of the disc. The compact disc player attempts to accomplish a constant linear speed by altering the rotational speed of the compact disc by means of a feedback servo system which uses the laser signal as part of the speed control system. Elaborate speed control systems have been developed to attempt to assure that a constant linear speed is maintained. Examples of such servo systems are shown in U.S. Patent No. 4,338,683, issued to Furukawa et al., U.S. Patent No. 4,539,667, issued to Fujiie, and U.S. Patent No. 4,611,319, issued to Naito. Each of these systems senses a synchronization signal from the compact disc playback and uses the signal through a servo mechanism to control the speed of the spindle motor driver. While such systems are generally effective, they require that the compact disc rotate at exactly the same speed as the spindle. Any variation in speed between the compact disc and the spindle, such as slippage, causes the servo mechanism to change the speed of the spindle, which, in turn, can affect the slippage. Compact discs are made of rigid, smooth, transparent plastic. During playback, the compact disc rests on a spindle which rotates during playback. The spindles on which compact discs rest are generally polished or finely machined hard materials, such as metal or plastic. The combination of the plastic disc and the smooth, polished spindle results in a possibility of the disc slipping with respect to the spindle during playback. Such slippage is particularly likely when the speed of the spindle changes. The macrolinear speed of the compact disc is closely controlled in the order of a few hundred revolutions per minute. In achieving a constant linear speed, angular velocity is changed during playback by the servo speed control system. There may be dislocations associated with the servo speed control. The disc changes speed during rotation play from the faster angular to the slower angular velocity in order to have a constant linear speed. This requires frequent micro changes or slowings of the angular or actual rotational speed expressed in revolutions per minute. Compact discs' angular inertia will tend to be greater than that of the spindle or drive mechanism. Micro slowings determined by the servo system of control to achieve constant linear velocity can result in a braking of the drive, which braking the inertia of the compact disc will try to overcome and result in slippage of the compact disc during play, and this slippage can generate vibration of the compact disc. There is a need for enhancing the playback characteristics or properties of compact discs by providing means to give good adhesion of the compact disc to the spindle on which it rests when it is rotated during playback. Not only is a frictional engagement enhancement needed, but there is also a need for an energy absorption means for compact discs which will allow the vibrational energy associated with slippage (or braking as noted) to be absorbed or dampened and not transmitted to the compact disc. DE-U-8504837 discloses a compact disc having an adhesive layer and an elastoplastic film layer covering one side of the disc to reduce vibration. JP-A-60256934 discloses a data storage disc for being read in a player having a spinning mechanism, the data storage disc comprising a pulse code modulated optical disc having a first side and a second side, the optical disc having a data storage portion on the first side and having a drive portion on the first side for engagement by the spinning mechanism, said disc comprising a friction component on a portion of the first side of the optical disc, the component substantially covering the drive portion and leaving the data storage portion completely uncovered. US-A-4 726 007 discloses a compact disc having an adhesive layer and a film adhered by said adhesive for suppressing and dissipating vibrational energy. The present invention overcomes the disadvantages of the prior art, and fills the need for enhancing the frictional engagement between compact discs and the spindle drive as well as the need for providing a vibrational damper which will absorb minute vibrations caused as the speed of the compact disc is changed or slowed during playback. The present invention is disclosed in appended claim 1. The present invention enhances the playback characteristics or properties of compact discs by providing good adhesion between the compact disc and the spindle on which it rests when it is rotated during playback. The present invention not only provides this frictional engagement enhancement, but also incorporates energy absorption characteristics into the frictional coating surface, of soft material, allowing the vibrational energy associated with slippage (or braking as noted) to be absorbed or dampened by the soft material and not transmitted to the compact disc. These and other advantages are achieved by the compact disc playback enhancer of the present invention. The friction component may be an annular ring, which is adhered to the disc. The ring may be adhered to the side of the disc containing the data storage portion. FIG. 1 is a perspective view of a compact disc showing the application of the frictional element of the playback enhancer of the present invention; and FIG. 2 is a perspective view of the other side of a compact disc such as that of FIG. 1, showing the dampening or vibration reducing element of the playback enhancer of the present invention. Referring more particularly to FIGS. 1 and 2, there is shown the compact disc playback enhancer of the present invention. A conventional compact disc 10 has a center opening 11 for engagement with the spindle of the compact disc player. The disc 10 has a lower playback surface 12 adapted to be optically read by the laser in the compact disc player, and a upper backing or nonplaying surface 13 which ordinarily contains a label identifying the disc. A typical compact disc for audio use has an outer diameter of 4.69 inches (120 cm) and a thickness of 0.056 inches (1.4 mm) with a center opening having a diameter of 0.58 inches (14 mm). The present invention contemplates coating the area of the compact disc that touches or contacts the spindle with a high friction component or coating, such as rubber, expanded polystyrene, silicone rubber, soft vinyl, styrene-diene polymer, or acrylic polymer, or with a tackiness agent or semi-adhesives such as used in removable labels or note pads. For instance, this area of the compact disc may be coated with a liquid plastisol which solidifies to form a soft, high-friction coating. Similarly, a film of material with desirable high friction can be adhered to the compact disc, such as adhering to the disc a thin layer of rubber, expanded polystyrene, or soft vinyl. Also, for instance, a paper or plastic film may be adhered to the compact disc or other film which has on its exposed surface a layer of rubber, expanded polystyrene, or other material that adheres well to the spindle, e.g., a self-adhesive ring that adheres tightly to the compact disc, and whose outer surface has a coating or film of material that adheres well to the spindle but which is easily separated from the spindle when removing the compact disc. Such a coating improves the playback characteristics or properties of the compact disc by providing improved adhesion of the compact disc to the spindle on which it rests when it is rotated during playback. In one embodiment of the present invention, as shown in FIG. 1, this friction component takes the form of an annular ring or disc 14 applied to lower playback surface 12 of the compact disc 10 around the opening 11 where the compact disc normally engages the spindle of the compact disc player. The ring 14 may have an outer diameter of between 1.25 inches (32 mm) and 1.50 inches (38 mm) with a center hole of 0.58 inches (14.7 mm) to 0.75 inches (19 mm) in diameter. The thickness of the ring 14 should be between 0.0002 inches (0.005 mm) and 0.0084 inches (0.2 mm). It is important that the frictional component be applied between the spindle and the compact disc. The frictional component can be adhered to the compact disc as just described, or, in the alternative, the frictional component can be adhered to the spindle so that the spindle has better adhesion to the compact disc during play. When the frictional component is placed between the spindle and the compact disc, the nature of the frictional component is such that it will adhere to both the spindle and the compact disc. The ring 14 comprising the frictional component of the present invention has been described as being adhered to the lower surface or playback surface 12 of the compact disc because this is the side of the compact disc that is engaged by the spindle of all or most existing conventional compact disc players. It is conceivable that the design of compact disc players could be changed or modified so that compact discs could be driven from the upper side of the disc instead, in which case the ring should be applied to the upper surface of the compact disc. The ring should be small enough that it does not interfere with the reading of the playback surface of the compact disc by the compact disc player. The geometry of the optical path of the laser reading the compact disc in the compact disc player is important. The distance between the spindle to the portion of the compact disc containing information is predetermined and is standard between various compact disc players in order to allow uniformity in playing of compact discs. This distance has allowable tolerances since various players are not identical. Therefore, the outer diameter of the ring must be less than the maximum possible distance to the playback portion of the compact disc, so that the player will function and read the disc properly. Materials that have been found to be beneficial for use as the frictional component include expanded polystyrene film, rubber film, polyvinyl chloride film, polyester (Mylar) film, silicone rubber film, paper, coated paper, and the like. Specific combinations that are suitable for use as a frictional component include expanded polystyrene film, which may be simply a ring of film laid on the spindle or a ring of film adhered to the central area of the compact disc, so that the ring does not interfere with the playback of the compact disc. Another possibility is a vinyl film adhered to the compact disc or a vinyl film coated with silicone rubber adhered to the compact disc. Other possibilities include paper adhered to the compact disc, polyester film adhered to the compact disc, or rubber-coated paper adhered to the compact disc. Such frictional components could be applied during the manufacture of the compact disc, either with the use of adhesives or by the use of mechanical means. In addition to their frictional properties, components adhered to the compact disc may also provide the damping or vibration-reducing potential capability. Acoustically, it has been found that the best performing materials have been softer ones, and the ones with the highest dynamic friction coefficients as estimated by subjective measurement. This measurement of dynamic friction coefficient for a compact disc showed a low coefficient of friction or tendency for the compact disc to slip easily over metal or plastic. Effective materials, as noted above, had significantly higher dynamic coefficients of friction than the compact disc. While the use of the ring 14 as the frictional component is preferred, other forms of frictional components can be used. For example, the frictional component may comprise a series of separate dots of a frictional material, or the component may comprise a number of small strips of frictional material arranged around the opening. Other forms for the frictional component are possible. The playback characteristics or properties of the compact disc may be further improved by providing means for dampening or reducing any tendency for vibration of the compact disc. This other aspect of the present invention involves the function of dampening or altering the vibrational potential of the compact disc. This can be accomplished, as shown in FIG. 2, by coating all or part of the upper backing or non-playing surface 13 of the compact disc with an energy-absorbing film layer or upper disc 15 that reduces the ability of the compact disc to vibrate. For example, such a coating layer 15 may be applied by covering the upper backing surface 13 of the compact disc with a liquid or emulsion which dries or cures to form an adherent elastic film over the upper surface of the compact disc. Vinyl plastisols which dry to form clear, soft, elastic films could be used. Alternatively, the upper surface 13 of the compact disc may be coated with a self-adhesive, flexible plastic film, self-adhesive paper, or coated paper to form the upper layer 15. The desirable properties of the upper layer 15 are that the layer be relatively thin, that the layer be self-adhesive or adhesive to the compact disc, that the layer be transparent, and that the layer be low mass. A film of flexible, non-rigid material may be adhered to the upper surface 13 of the compact disc to form the upper layer 15. Such films may be clear, transparent, colored, or opaque. For practical purposes, the film used for the upper layer 15 should be clear so that the label on the upper surface 13 of the compact disc is still visible. The film should also be relatively thin to avoid interfering with the rotation of the compact disc. Effective films can be as thin as 0.001 inch (0.025 mm) and can be up to 0.0035 inch (0.09 mm) or thicker. The use of an adhesive film of low mass is desirable because its mass is sufficiently low that if it is not centered precisely, it will not affect the moment of inertia or rotation of the compact disc. Preferably, the mass of the upper layer 15 is less than about 0.1 ounces (3 grams). Since compact discs are approximately 0.056 inches (1.4 mm) thick, it can be seen that effective films for adhering to the top of the compact discs are very thin compared to the thickness of the disc. Suitable films for this purpose include polyvinyl chloride film, commonly found in Contact brand self-adhesive films, or polyvinyl chloride film such as that available from Avery International, Fasson Specialty Materials Division, Painesville, Ohio, U.S.A., in a thickness of 0.0025 inches (0.06 mm), and polyester film in a thickness of 0.002 inches (0.05 mm) from Fasson, or from Tekra Corporation, New Berlin, Wisconsin, U.S.A. The upper layer 15 may be in the form of a disc having an outer diameter of between 4.5 inches (115 mm) and 4.690 inches (120 mm) with a center hole of 0.58 inches (15 mm) to 0.75 inches (19 mm) in diameter. The thickness of the upper disc 15 should be between 0.0005 inches (0.01 mm) and 0.20 inches (5 mm), and preferably between 0.002 inches (0.05 mm) and 0.0045 inches (0.11 mm) The upper disc 15 may be as large as the compact disc (or other optical disc) to which it is adhered, or it may be smaller than the compact disc to which it is adhered. There is a practical limit for the smallest size of the upper disc in order to achieve the dampening or vibration reducing advantages of the present invention. As shown in FIGS. 1 and 2, a combination of both layers 14 and 15 may be applied to the compact disc. The combination produces synergistic results and greatly improves the compact disc playback results, and provides outstanding improvement in the quality of the compact disc playback. A particularly excellent combination is the application by means of an adhesive layer of a 0.0025-inch (0.06-mm) ring of cast vinyl film, available from Fasson, to the lower playback side of a compact disc with a 0.002 inch (0.05 mm) flexible polyester film available from Tekra to the upper backing side of the compact disc. The following is a list of suitable flexible materials which are appropriate for making the layers to adhere to either side of the compact disc: The softness of the materials in above table is described in terms of Shore hardness. The Shore A scale is used for relatively soft materials such as rubbers, and therefore is appropriate for the purpose of describing the materials herein. The Shore method measures the pressure needed to indent a relatively thick layer of material, on the order of ⅛ inch (3 mm). Since the materials being used are very thin in combination with an adhesive that does not harden and remains as an extremely viscous liquid, even the relatively harder materials are usable because of their thinness and, therefore, their flexibility. The adhesive used to adhere the ring 14 onto the lower playback side of the compact disc and to adhere the disc layer 15, where required, onto the upper backing (non-playing) side of the compact disc, contributes to the dampening of the disc. The thickness and nature of the adhesive will determine the amount of dampening provided by the adhesive. Adhesives employed are polymeric and chemically related to several of the species mentioned in the table above. The adhesives can be likened to plastics or rubbers of extreme softness or penetrability. Typical adhesives are rubber-based or rubberlike materials, such as natural rubber or silicone rubber or acrylic ester polymers, or similar polymers. They can be considered extremely viscous liquids. The adhesives involved are non-rigid and flexible or extremely soft but coherent liquids. Non-rigid polymeric adhesives such as those based on rubber or acrylic polymers and the like have some of the properties of elastomers, such as flexibility, creep resistance, resilience, and elasticity, and do provide useful dampening to enhance the quality of playback of compact discs. The chemistry of non-rigid polymeric adhesives is diverse, and includes polymers of the types of materials described herein as elastomers and rubbers, as flexible thermoplastics, and as thermoplastic elastomers. For the purposes of this invention, the appropriate adhesives are included within the above-listed definitions of elastomers and rubbers, flexible thermoplastic, thermoplastic elastomers and elastomeric alloys. EXAMPLE 1 An example of a compact disc playback enhancer of the present invention involves the preparation and use of a single annular ring or disc having three layers, applied to the lower surface or playback surface of a compact disc. The first layer or outer layer of the ring is a 1.5-mil (0.0015-inch) (0.038-mm) thick film of silicone rubber. Suitable silicone rubber is available from General Electric Company, Silicone Products Division, Waterford, New York, U.S.A. This first layer is self-adhered to a second layer or substrate, which is a 3.2-mil (0.0032-inch) (0.081-mm) thick layer of paper. The third layer or adhesive layer is a 1.5-mil (0.0015-inch) (0.038-mm) thick layer of rubber adhesive which is used to adhere the ring to the compact disc. Such a paper plus rubber adhesive layer is marketed as a product called Avery Self-Adhesive Label by Avery Label Company, Chicago, Illinois, U.S.A. The combined thickness of the three-layer ring is 6.2 mils (0.0062 inches) (0.157 mm), and the ring has an outer diameter of 1.25 inches (32 mm) and a central opening of 0.66 inches (17 mm) in diameter. The ring prepared in accordance with this example is centered and adhered to the lower surface or playback surface of a compact disc using the rubber adhesive of the ring. When the compact disc was played in a conventional compact disc player, the addition of the ring resulted in a very major improvement in acoustic quality over the compact disc without the ring. EXAMPLE 2 Another example of a compact disc playback enhancer of the present invention involves the preparation and use of both an annular ring applied to the lower surface or playback surface of a compact disc and a film layer applied to the upper backing surface of the compact disc. The annular ring has three layers, and is essentially identical to that described above in Example 1. The upper film layer is a disc of a 2.0-mil (0.0020-inch) thick, colorless, transparent polyester film coated with a 1.0-mil (0.0010-inch) (0.025-mm) thick, colorless, transparent layer of flexible acrylic polymer adhesive. A suitable product that combines the polyester film with acrylic polymer adhesive is marketed as a self-adhesive overlaminating film designated FasCal Super Cold Seal by Avery International, Fasson Specialty Division, Painesville, Ohio, U.S.A. The resulting disc is 3.0 mils (0.0030 inches) (0.076 mm) thick, and has an outer diameter of 4.5 inches (115 mm) and a central opening of 0.66 inches (17 mm) in diameter. The ring prepared in accordance with this example is centered and adhered to the lower surface or playback surface of a compact disc using the rubber adhesive of the ring. The upper disc prepared in accordance with this example is centered and adhered to the upper backing surface of the compact disc using the acrylic polymer adhesive of the upper disc. When the compact disc was played in a conventional compact disc player, the addition of the ring and the upper disc resulted in a major improvement in acoustic quality over the compact disc with the ring alone as described in Example 1. EXAMPLE 3 Another example of a compact disc playback enhancer of the present invention involves the preparation and use of a different annular ring applied to the lower surface or playback surface of a compact disc with a film layer applied to the upper backing surface of the compact disc. The annular ring has four layers. The first layer or outer layer of the ring is a 2.5-mil (0.0025-inch) (0.06-mm) thick layer of vinyl film. The second layer is a 1.0-mil (0.0010-inch) (0.025-mm) thick layer of flexible acrylic polymer adhesive. A suitable vinyl film combined with a layer of acrylic polymer adhesive is marketed as FasCal Durable Film 908 by Avery International, Fasson Specialty Division, Painesville, Ohio, U.S.A. The second layer is used to adhere the first or outer layer to a third layer or substrate, which is a 2.0-mil (0.0020-inch) (0.05-mm) layer of polyester film. The fourth or adhesive layer is a 1.0-mil (0.0010-inch) (0.025 mm) layer of flexible acrylic polymer adhesive which is used to adhere the ring to the compact disc. A suitable product that combines the polyester film with acrylic polymer adhesive is marketed as a self-adhesive overlaminating film designated FasCal Super Cold Seal by Avery International, Fasson Specialty Division, Painesville, Ohio, U.S.A. The combined thickness of the four-layer ring is 6.5 mils (0.0065 inches) (0.16 mm), and the ring has an outer diameter of 1.25 inches (32 mm) and a central opening of 0.66 inches (17 mm) in diameter. The upper film layer is essentially the same as the upper disc described in Example 2 above. The ring prepared in accordance with this example is centered and adhered to the lower surface or playback surface of a compact disc using the acrylic polymer adhesive of the ring. The upper disc prepared in accordance with this example is centered and adhered to the upper backing surface of the compact disc using the acrylic polymer adhesive of the upper disc. When the compact disc was played in a conventional compact disc player, the addition of the ring and the upper disc resulted in a very major improvement in acoustic quality over the original compact disc. These examples apply to conventional audio compact discs as described above having an outer diameter of 4.69 inches (120 mm) and a thickness of 0.58 inches (15 mm), with a center opening of 0.056 inches (1.4 mm) in diameter. Of course, the dimensions of the rings and upper discs described in the examples above could be modified for use on other optically encoded playback discs, such as "mini" audio compact discs or video discs. The invention has been described with respect to compact discs used for audio reproduction, but the same concept can be used with other discs which are rotated and used to record data or information that can be read, such as words or numbers, or which include visual recordings, printed or graphic materials, or other data. Furthermore, the invention has applicability to storage media analogous to compact discs, and which provide read/write capabilities rather than read-only recording.

Claims: A data storage disc for being read in a player having a spinning mechanism, the data storage disc comprising a pulse code modulated optical disc (10) having a first side (12) and a second side (13), the optical disc having a data storage portion on the first side (12) and having a drive portion on the first side (12) for engagement by the spinning mechanism, said disc comprising: an annular ring friction component (14) provided on a portion of the first side (12) of the optical disc (10), the component (14) substantially covering the drive portion and has a friction coefficient greater than that of the optical disc (10) and leaving the data storage portion completely uncovered; characterised in that said component (14) has a thickness of between 0.005 mm and 0.2 mm, for providing absorption and dampening of vibrations of the optical disc and preventing vibrations of the spinning mechanism from being transmitted to the optical disc and improving the frictional engagement of the optical disc with the spinning mechanism, and has a Shore A hardness of between 15 and 100; (14) and that said component (14) is adhered to the first side portion of the optical disc with a flexible adhesive layer being non-rigid and pliable and made from a soft flexible material for absorbing or dampening vibrations and preventing the vibrations from being transmitted to optical disc having a thickness of between 0.025 mm and 0.038 mm. A data storage disc as defined in claim 1, wherein the material of the component (14) is substantially softer and more flexible than that of the optical disc (10), and selected from the group consiting of elastomers and rubbers, flexible thermoplastics, thermoplastic elastomers, elastomeric alloys paper and mixture thereof. A data storage disc as defined in claim 1, comprising in addition: a component layer (15) on the second side (13) of the optical disc, the component layer (15) being a dampening and friction component layer; and a second adhesive layer means for adhering the component layer (15) onto the second side of the optical disc.

Inventors: MURPHY JOHN P, MURPHY, JOHN P.
Applicants: MURPHY JOHN P, MURPHY, JOHN P.
Application year: 1989
Main classification: G11B7
IPC codes: G11B7, G11B23