GB1601448A - Narrowed-electrode pcikup stylus and forming methods for video disc systems - Google Patents

Description

(54) NARROWED-ELECTRODE PICKUP STYLUS AND FORMING METHODS FOR VIDEO DISC SYSTEMS (71) We, RCA CORPORATION. a corporation organized under the laws of the State of Delaware, United States of America, of 30 Rockefeller Plaza, City and State of New York, 10020, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a disc record playback system of a form suitable for playback of high density information records such as video discs, and, in particular, a stylus structure and a method for forming the stylus structure advantageous for use in such playback systems.

In U.S. Patent. No. 2.842,194 issued on October 15, 1974 to Jon K. Clemens, a video playback system of a variable capacitance form is disclosed. In one configuration of the Clemens system, information, representative of recorded picture and sound, is encoded in a relatively fine spiral groove on the surface of a disc record (e.g., groove width 3.5 micrometers, groove depth - 1.0 micrometers). The record substrate is covered with a coating of conductive material and a dielectric layer overlying the conductive coating. During playback, a pickup stylus, having a thin conductive electrode (e.g., about 0.2 micrometers thick), engages the groove as the record is rotated by a supportive turntable. Capacitive variations between the stylus electrode and the conductive coating are sensed to recover the prerecorded information.

In the systems of the above type, the use of a relatively fine record groove and the groove-engaging requirement for the pickup stylus result in a stylus tip which is extremely small.

Typically, the stylus tip is defined by a prow, a substantially flat, V-shaped rear surface remote from the prow, a bottom surface extending from the bottom edge of the rear surface and a pair or side surfaces and converging at the prow. The intersection of the rear and the side surfaces with the bottom surface forms, for example, a triangular footprint. The entire rear surface is covered with conductive material to form a V-shaped electrode. Illustratively, the angle included between the two side edges is equal to 42", the length of the bottom edge is about 2 micrometers, the height of the triangular footprint is about 4 micrometers, the angle between the prow and the rear surface is about 40 , and the electrode depth is about 0.2 micrometers.

In the previously mentioned Clemens U.S. patent, two alternative forms of stylus structure are disclosed: (1) a symmetrical structure, wherein the conductive stylus electrode is "sandwiched" between dielectric support material, which extends ahead of, and behind, the electrode in symmetrical fashion (Figure 1); and (2) an "asymmetrical" structure, wherein the stylus electrode comprises a conductive coating on the rear surface of a dielectric support (Figure 5).

The asymmetrical stylus form has proven to be convenient for ease in stylus fabrication, in that the entire rear surface is coated with conductive material. However, the stylus electrode, being V-shaped, gets wider as the stylus structure wears away. As the stylus electrode gets wider, the spurious capacitance between the electrode and the conductive coating in the adjacent grooves becomes significant, introducing noise in the recovered signal. When the stylus electrode becomes wider than the groove width (e.g., 3.5 micrometers), it is desirable to replace the stylus.

Although the sandwich form of the stylus structure overcomes the problem of increasing cross-talk with the increase in the width of the stylus electrode, it is difficult to fabricate. In particular, it is difficult to achieve a good registration (ie., position, attitude, etc.) between the stylus electrode and the stylus tip. A pickup stylus which overcomes the above-mentioned problems, and yet which is relatively easy to fabricate, would be desirable.

According to a first aspect of the present invention there is provided a pickup stylus for a playback system including a turntable for rotating a disk record having signals prerecorded along a spiral track of a given width and disposed on the surface of said record; said pickup stylus playing back said prerecorded signals when stylus/record relative motion is established; and comprising: a dielectric support element which tapers towards a tip; said dielectric support element having an arrowhead-shape record-engaging portion in the region of said tip; said arrowhead-shape record-engaging portion being defined by a rear edge, a pair of said edges extending substantially orthogonally from the ends of said rear edge, a pair of outwardly extending edges originating from the ends of said side edges remote from said rear edge, and a pair of edges extending from the ends of said outwardly extending edges remote from said edges and converging at a prow; said dielectric support element further including in said tip region thereof a substantially flat rear surface angularly disposed relative to said record-engaging portion remote from said prow and having a pair of substantially parallel edges terminating at said ends of said rear edge; the maximum separation between said substantially parallel edges of said angularly disposed rear surface, in the region of said tip. being less than said given track width; and a layer of conductive material adherent to said angularly disposed rear surface between said substantially parallel edges thereof to form an electrode; wherein said stylus is arranged in said track for playback such that, at the point of reception of said stylus, said rear edge is disposed transverse to said track and in contact therewith, said substantially orthogonally extending side edges of said recordengaging portion are disposed in the direction of said track and the movement of said record beneath said stylus reception point extends from said prow toward said rear edge.

The invention in a second aspect provides a method of fabricating a narrowedelectrode stylus by modifying a tapering dielectric support element having a tip which is defined by a prow and a substantially flat V-shaped rear surface remote from said prow: the entire rear surface of said dielectric support element being covered with conductive material to form an electrode; said narrowed-electrode stylus being suitable for playing back prerecorded signals from a disc record groove of a given width and a given pitch; said method comprising the steps of: introducing said tip of said tapering support element in an abrasive groove having a bottom wall, side walls separated by a distance less than said given width and lands extending away from said side walls; while establishing relative motion between said tapering support element and said abrasive groove along a line which is disposed in a plane substantially perpendicular to said V-shaped rear surface; gently tilting said electrode face of said support element toward said abrasive groove until it subtends a shallow angle with the bottom wall of said abrasive groove to form a portion having an arrowhead-shape cross-section in the region of said tip; the apex of said arrowhead-shape crosssection being located on said prow; the base of said arrowhead-shape cross-section being defined by said substantially flat rear surface; said lands of said abrasive grooves forming the shoulders of said arrowhead-shape cross-section; said side walls of said abrasive groove forming the sides of said arrowhead-shape cross-section interconnecting said shoulders and said base of said arrowhead-shape cross-section; and terminating said relative motion subsequent to formation of said tip portion having an arrowhead-shape cross-section.

In the drawings: Figure 1 illustrates a video disc system for playing back prerecorded signals recorded in a disc record groove of a given width in which a narrowed-electrode stylus constructed in accordance with the present invention may be advantageously employed; Figure 2 illustrates an asymmetrical form of a prior art pickup stylus; Figures 3 and 4 depict successive steps in the life of the asymmetrical stylus of Figure 2, the asymmetrical stylus being shown riding in a disc record groove; Figure 5 shows a sandwich form of a pickup stylus structure; Figure 6 illustrates a perspective view of a narrowed-electrode pickup stylus structure, in accordance with the principles of the present invention, suitable for use in the system of Figure 1; Figures 7 and 8 are respectively the front view and the side view of the narrowedelectrode pickup stylus of Figure 6; Figures 9-14 are cross-sections of the narrowed-electrode pickup stylus of Figures 6-8 taken along lines 9-9 to 14-14, respec tively, in Figure 7; Figures 15 and 16 illustrate a portion of lapping disc suitable for forming the narrowed-electrode pickup stylus of Figures 6-8; and Figure 17 shows the narrowed-electrode pickup stylus of Figures 6-8 before stylus shoe lapping.

Referring to Figure 1. there is illustrated therein a video disc system 20 which is illustratively of a variable capacitance type as generally shown in the aforesaid Clemens patent. The system 20 includes a turntable 22 rotatably mounted on a base 24 for rotation at a proper playback speed (e.g., 450 rpm). The turntable 22 supports and centers a video disc 26 having information, representative of picture and sound. recorded along a sprial groove disposed on the surface thereof. To obtain an adequate playback time, the groove convolutions on the video disc 26 are relatively closely spaced (e.g., groove pitch - 3.5 micrometers). As stated previously, the video disc 26 has a dielectric deposit overlying a conductive coating on the video disc substrate.

A stylus arm 28, carrying a grooveengaging pickup stylus 30 at the free end thereof, is pivotally supported at its remote end to a carriage 32. The stylus arm pivot is, for example, of the type disclosed in U.S.

Patent No. 3,917.903 issued to B.K. Taylor, et al.

To enable the pickup stylus 30 to maintain a desired constant attitude in the video disc groove, the carriage 32 is traversed radially of the video disc 26 during playback at a speed suitably synchronized with the rotation of the video disc. The carriage traversing system is, for example, of the type shown in U.S. Patent No. 3,870,835 issued to F.R. Stave.

As previously indicated, the pickup stylus 30 incorporates a relatively thin electrode (e.g., 0.2 micrometers deep). The capacitance variations between the stylus electrode and the video disc conductive coating are sensed during playback by a pickup circuit 34. The output of the pickup circuit 34 is processed by a signal processing circuit 36 to a form suitable to be used by a television receiver. The pickup circuit 34 is, for example, of the type illustrated in our U.K. Specification No. 1574594 (inventors Kawamoto and Denlinger). The signal processing circuit 36 is, for example. of the type illustrated in U.S. Patent No. 3,969.757 issued to J. Amery.

Figure 2 shows an asymmetrical prior art pickup stylus having a V-shaped tip. The asymmetrical stylus 38 includes a dielectric support element 40 which has a V-shaped tip 42. The terminal portion of the V-shaped tip 42 is defined by a prow 44, a substantially flat rear surface 46 remote from the prow, a bottom surface 48 extending from the bottom edge 50 of the rear surface and a pair of side surfaces 52 and 54 extending, respectfully, from the side edges 56 and 58 of the rear surface and intersecting at the prow.

The intersection of the rear surface 46 and the side surfaces 52 and 54 with the bottom surface 48 defines for example, a triangular stylus footprint. The entire rear surface 46 of the stylus support element 40 is coated with conductive material to form an electrode 60.

Figures 3 and 4 show successive stages in the life of the asymmetrical stylus 38 riding in a groove disposed on the surface of a video disc 62 having a dielectric deposit 64 overlying a conductive coating 66 on the video disc substrate 68.

As the asymmetrical stylus 38 wears to a shape shown in Figure 4, the width of the stylus electrode 60 becomes the same as the distance between the adjacent grooves (e.g., 3.5 micrometers). When the stylus electrode 60 becomes wider, the cross-talk in the signal pickup output increases and it becomes desirable to replace the pickup stylus.

Figure 5 shows a sandwich form of a pickup stylus structure 70. As shown therein, the sandwich stylus includes a first and second dielectric support members 72 and 74 in which a narrow conductive electrode 76 is embedded.

As previously indicated, although the sandwich stylus is advantageous in that the stylus electrode does not get wider as the stylus wears away, it is difficult to achieve an accurate registration between the stylus electrode and the stylus tip due to the smallness of the dimensions involved.

A narrowed-electrode pickup stylus 80 having an arrowhead cross-section in the region of its tip reduces these problems.

Figure 6 is a perspective view of the narrowed-electrode stylus 80. Figures 7 and 8 illustrate, respectively, the front view and the side view of the narrowed-electrode stylus 80. Figures 9-14 show successive cross-sections of the narrowed-electrode stylus.

As shown in the respective drawings, the narrowed-electrode stylus 80 includes a dielectric support element 82 which tapers toward a tip 84. The dielectric support element 82 has an arrowhead cross-section in the region of its tip 84.

The arrowhead-shape cross-section is defined by a prow 86, a substantially flat rear surface 88 remote from the prow having substantially parallel edges 90 and 92, a pair of side surfaces 94 and 96 converging toward the prow, a pair of additional side surfaces 98 and 100 extending substantially orthogonally from the edges of the rear surface and extending toward the prow, and shoulder surfaces 102 and 104 joining the orthogonal surfaces with the converging surfaces.

The entire rear surface of the dielectric support element 82 is coated with conductive material to form a narrowed-electrode 106.

The dielectric support element 82 further has a bottom surface 108 extending from the bottom edge 110 of the substantially flat rear surface 88. The intersection of the rear, the orthogonal, the shoulder and the converging surfaces with the bottom surface 108 defines a stylus foot-print. In this particular embodiment, the stylus footprint has an arrowhead-shape.

It can be seen from Figures 9-13 that the stylus electrode 106 does not get wider as the stylus wears away. Therefore, the signalto-noise ratio obtained by using the arrowheaded stylus is fairly constant throughout its useful life.

Additionally. it can be seen that the stylus life is appreciably extended since a greater volume of the stylus structure has to wear away before the stylus becomes unusable.

An additional reason for the increase in stylus life is the fact that the stylus electrode is located in a relatively protected area during playback.

Illustratively, the configuration of the narrowed-electrode stylus 80 is as follows: the angle between the bevelled surfaces 94 and 96--42", the length of the bottom edges 110 of the narrowbead shape footprint--3 micrometers, the longitudinal dimension of the arrowhead shape footprint--8 micrometers, the length of each of the shoulders of the arrowhead shape footprint--1 micrometer. the longitudinal dimension of the base of the arrowhead shape footprint --2 micrometers, the height of the narrowedelectrode in the region of stylus tip --5 micrometers, the electrode depth --0.2 micrometers, and the angle between the prow and the bottom surface 108 of the stylus 1450.

With this configuration. the life of the narrow-electrode stylus is about twice the life of the previously mentioned V-tipped stylus and the increment in the signal-tonoise ratio is of the order of 3dB (average).

The narrowed-electrode stylus not only offers advantages in performance and stylus life, but it also is relatively easy to fabricate by modifying a V-tipped stylus to obtain a narrowed-electrode stylus.

The narrowed-electrode stylus 80 is formed by introducing a V-tipped stylus 112 into an abrasive groove 114 of a lapping disc 116 with its electrode 118 disposed substantially orthogonally to the lapping disc, as shown in Figure 15, while an appropriate stvlusvdisc relative motion is established.

After allowing the stylus 112 to register itself with respect to the groove 114, the electrode face of the stylus is gently tilted toward the lapping disc 116 until the electrode 118 makes a substantially shallow angle relative to the lapping disc (e.g., 5 ) as shown in Figure 16. The stylus 112 is run in the tilted position for a time interval appropriate for the dimensions desired (e.g., 5-20 seconds where the dielectric support element is sapphire and the lapping disc has SiO2 coating).

Figure 17 shows the way the stylus 112 looks after it is run in the lapping disc 116 in the tilted position for the specified time interval. As shown therein, the shoulder surfaces 120 and 122 are lapped by the lands 124 and 126 of the lapping disc 116 and the substantially orthogonal surfaces 128 and 130 are lapped by the walls 132 and 134 of the abrasive groove 114. The bevelled surfaces 136 and 138 of the stylus 112 intersect with each other to define a prow 140.

The bottom of the stylus 112 shown in Figure 17 is shoe-lapped to form a substantially flat bottom surface. After shoelapping, the stylus will be of the form shown in Figures 6-8.

The narrowed-electrode stylus fabricated in this fashion provides a very precise registration between the narrowedelectrode and the stylus tip.

While in this particular embodiment, the abrasive groove 114 has a trapezoidal crosssection, as shown in Figure 15, to obtain substantially orthogonal surfaces 128 and 130, the abrasive groove may be made to have a V-shaped cross-section to obtain flared surfaces interconnecting the shoulder surfaces 120 and 122 with the rear surface of the stylus support element.

The width of the abrasive groove 114 is determined by the desired width for the narrowed electrode (e.g., 3 micrometers).

The lapping disc 116 shown in Figure 15 has lands 124 and 126 which extend substantially orthogonally from the walls 122 and 124 of the abrasive groove 114 to obtain shoulder surfaces 120 and 122 as shown in Figure 17. However, the lands 124 and 126 may be flared so as to obtain flared shoulder surfaces interconnecting the orthogonal surfaces with the converging surfaces of the stvlus.

The V-tipped stylus 112 may be held in orthogonal position shown in Figure 15 for a given time interval to obtain a keel tip of the type featured in our U.S. Patent Application No. 4104832 issued in the name of E.O.

Keizer, and entitled "Method for forming keel-tipped stylus for video disc systems Subsequent to the formation of the keel tip, the electrode face of the stylus 112 can be gently tilted toward the lapping disc until it subtends an acute angle relative to the lapping disc 116. The stylus 112 may be held in the tilted position shown in Figure 16 for a specified time interval to form a section having an arrowhead-shape cross-section above the keel tip. The stylus made in this fashion will have a pentagonal footprint until the keel tip is worn away, and then it will have an arrowhead shape footprint.

The manufacture of the lapping disc 116 will now be described. A fairly flat and smooth copper clad substrate is coated with a thick coating (e.g., several micrometers) of photoresist. the photoresist coated substrate is exposed with an intense beam of energy (e.g., electron beam or light) of the desired width (e.g., 3.0 micrometers) along a spiral track of a coarse pitch. After developing the exposed photoresist coated substrate, the exposed photoresist region is removed to form a coarse spiral groove on the substrate. A metal stamper is obtained therefrom by a process such as electroless plating. The metal stamper is used to press plastic discs. The plastic discs may be made from the same material as video discs. An abrasive material, such as SiO2 is deposited thereon to form an abrasive layer 142 as shown in Figures 12 and 13.

Narrowed-electrode styli, in accordance with the embodiments of the present invention, have been successfully made from diamond, as well as from sapphire, support elements.

WHAT WE CLAIM IS: 1. A pick up stylus for a playback system including a turntable for rotating a disk record having signals prerecorded along a spiral track of a given width and disposed on the surface of said record; said pick up stylus playing back said prerecorded signals when stylus/record relative motion is established; and comprising: a dielectric support element which tapers toward a tip; said dielectric support element having an arrowhead-shape record-engaging portion in the region of said tip; said arrowhead-shape record-engaging portion being defined by a rear edge, a pair of side edges extending substantially orthogonally from the ends of said rear edge, a pair of outwardly extending edges originating from the ends of said side edges remote from said rear edge, and a pair of edges extending from the ends of said outwardly extending edges remote from said side edges and converging at a prow; said dielectric support element further including in said tip region thereof a substantially flat rear surface angularly disposed relative to said record-engaging portion remote from said prow and having a pair of substantially parallel edges terminating at said ends of said rear edge; the maximum separation between said substantially parallel edges of said angularly disposed rear surface, in the region of said tip, being less than said given track width; and a layer of conductive material adherent to said angularly disposed rear surface between said substantially parallel edges thereof to form an electrode; wherein said stylus is arranged in said track for playback such that, at the point of reception of said stylus, said rear edge is disposed transverse to said track and in contact therewith, said substantially orthogonally extending side edges of said recordengaging portion are disposed in the direction of said track and the movement of said record beneath said stylus reception point extends from said prow toward said rear edge.

2. A pickup stylus as defined in Claim 1 wherein said dielectric support element exhibits said cross-section at its extremity so that said pickup stylus has an arrowheadshape footprint.

3. A pickup stylus as defined in Claim 1 wherein, in the region of said stylus tip, the ratio defined by the electrode width in the numerator and the maximum corresponding dimension of said support element in the denominator decreases in a direction opposite to the direction of said taper.

4. A method of fabricating a narrowedelectrode stylus by modifying a tapering dielectric support element having a tip which is defined by a prow and a substantially flat V-shaped rear surface remote from said prow; the- entire rear surface of said dielectric support element being covered with conductive material to form an electrode; said narrowed-electrode stylus being suitable for playing back prerecorded signals from a disc record groove of a given width and a given pitch; said method comprising the steps of: introducing said tip of said tapering support element in an abrasive groove having a bottom wall, side walls separated by a distance less than said given width and lands extending away from said side walls; while establishing relative motion between said tapering support element and said abrasive groove along a line which is disposed in a plane substantially perpendicular to said V-shaped rear surface; gently tilting said electrode face of said support element toward said abrasive groove until it subtends a shallow angle with the bottom wall of said abrasive groove to form a portion having an arrowhead-shape cross-section in the region of said tip; the apex of said arrowhead-shape crosssection being located on said prow; the base of said arrowhead shape cross-section being defined by said substantially flat rear surface; said lands of said abrasive grooves form

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. in the tilted position shown in Figure 16 for a specified time interval to form a section having an arrowhead-shape cross-section above the keel tip. The stylus made in this fashion will have a pentagonal footprint until the keel tip is worn away, and then it will have an arrowhead shape footprint. The manufacture of the lapping disc 116 will now be described. A fairly flat and smooth copper clad substrate is coated with a thick coating (e.g., several micrometers) of photoresist. the photoresist coated substrate is exposed with an intense beam of energy (e.g., electron beam or light) of the desired width (e.g., 3.0 micrometers) along a spiral track of a coarse pitch. After developing the exposed photoresist coated substrate, the exposed photoresist region is removed to form a coarse spiral groove on the substrate. A metal stamper is obtained therefrom by a process such as electroless plating. The metal stamper is used to press plastic discs. The plastic discs may be made from the same material as video discs. An abrasive material, such as SiO2 is deposited thereon to form an abrasive layer 142 as shown in Figures 12 and 13. Narrowed-electrode styli, in accordance with the embodiments of the present invention, have been successfully made from diamond, as well as from sapphire, support elements. WHAT WE CLAIM IS:

1. A pick up stylus for a playback system including a turntable for rotating a disk record having signals prerecorded along a spiral track of a given width and disposed on the surface of said record; said pick up stylus playing back said prerecorded signals when stylus/record relative motion is established; and comprising: a dielectric support element which tapers toward a tip; said dielectric support element having an arrowhead-shape record-engaging portion in the region of said tip; said arrowhead-shape record-engaging portion being defined by a rear edge, a pair of side edges extending substantially orthogonally from the ends of said rear edge, a pair of outwardly extending edges originating from the ends of said side edges remote from said rear edge, and a pair of edges extending from the ends of said outwardly extending edges remote from said side edges and converging at a prow; said dielectric support element further including in said tip region thereof a substantially flat rear surface angularly disposed relative to said record-engaging portion remote from said prow and having a pair of substantially parallel edges terminating at said ends of said rear edge; the maximum separation between said substantially parallel edges of said angularly disposed rear surface, in the region of said tip, being less than said given track width; and a layer of conductive material adherent to said angularly disposed rear surface between said substantially parallel edges thereof to form an electrode; wherein said stylus is arranged in said track for playback such that, at the point of reception of said stylus, said rear edge is disposed transverse to said track and in contact therewith, said substantially orthogonally extending side edges of said recordengaging portion are disposed in the direction of said track and the movement of said record beneath said stylus reception point extends from said prow toward said rear edge.

2. A pickup stylus as defined in Claim 1 wherein said dielectric support element exhibits said cross-section at its extremity so that said pickup stylus has an arrowheadshape footprint.

3. A pickup stylus as defined in Claim 1 wherein, in the region of said stylus tip, the ratio defined by the electrode width in the numerator and the maximum corresponding dimension of said support element in the denominator decreases in a direction opposite to the direction of said taper.

4. A method of fabricating a narrowedelectrode stylus by modifying a tapering dielectric support element having a tip which is defined by a prow and a substantially flat V-shaped rear surface remote from said prow; the- entire rear surface of said dielectric support element being covered with conductive material to form an electrode; said narrowed-electrode stylus being suitable for playing back prerecorded signals from a disc record groove of a given width and a given pitch; said method comprising the steps of: introducing said tip of said tapering support element in an abrasive groove having a bottom wall, side walls separated by a distance less than said given width and lands extending away from said side walls; while establishing relative motion between said tapering support element and said abrasive groove along a line which is disposed in a plane substantially perpendicular to said V-shaped rear surface; gently tilting said electrode face of said support element toward said abrasive groove until it subtends a shallow angle with the bottom wall of said abrasive groove to form a portion having an arrowhead-shape cross-section in the region of said tip; the apex of said arrowhead-shape crosssection being located on said prow; the base of said arrowhead shape cross-section being defined by said substantially flat rear surface; said lands of said abrasive grooves form

ing the shoulders of said arrowhead-shape cross-section; said side walls of said abrasive groove forming the sides of said arrowhead-shape cross-section interconnecting said shoulders and said base of said arrowhead-shape cross-section; and terminating said relative motion subsequent to formation of said tip portion having an arrowhead-shape cross-section.

5. A method as defined in Claim 4 further including the step of lapping said tapering support element to form a bottom surface substantially perpendicular to said rear surface subsequent to termination of said relative motion to form a stylus shoe.

6. A method as defined in Claim 4 wherein said abrasive groove forms a spiral groove on a lapping disc having a pitch coarser than said given pitch of said disc record.

7. A method of fabricating a pickup stylus substantially as hereinbefore described with reference to Figures 15-17.

8. A stylus substantially as hereinbefore described with reference to Figures 6-14.