Stylus for reproducing capacitive videodisc

A reproducing stylus traces a track of a recording medium, in which track an information signal is recorded as variations of geometrical shape, and reproduces as variations in capacitance the information signal thus recorded. The reproducing stylus comprises a reproducing stylus main structure having a tip part with a flat sliding contact face for slidingly contacting a plurality of track of the recording medium at a time, and an electrode secured to the reproducing stylus main structre at the tip part thereof and adapted to trace one track and reproduce the information signal as the variations in capacitance in accordance with the variations in geometrical shape. The electrode has over a specific distance over the tip part of the stylus main structure from the extreme tip thereof, a constant width in the width direction of the track which is less than the width of the track. The extreme tip of the electrode has a flat or linear tip end contacting the track.

BACKGROUND OF THE INVENTION 
The present invention relates generally to stylii for reproducing 
information signals recorded on information signal recording mediums. More 
particularly, the invention relates to a stylus for reproducing, as 
capacitance variation, an information signal recorded with high density as 
a variation in geometrical shape on an information signal recording medium 
of disc shape. 
As one of systems, in which information signals such as video signals are 
recorded with high density on recording mediums of disc shape and are 
reproduced, there is a system which has been developed and reduced to 
practice, in its recording system, pits are formed along a track in 
accordance with an information signal in a recording medium substrate 
structure, and an electroconductive layer is adhered onto this substrate 
structure thereby to form a recording medium on which recording of the 
information has been completed. Then, in the reproducing system, this 
recording medium is rotated, and a reproducing stylus having an electrode 
is caused to trace relatively over and along the track of the medium. As a 
result, the elestrostatic capacitance between the electrode of the 
reproducing stylus and the electroconductive layer of the recording medium 
varies in accordance with the pits, whereby the information signal is 
reproduced in response to the variation of the capacitance. 
Among the recording and reproducing systems of this so-called capacitance 
conversion type, there has been one in which, in its recording system, the 
pits are formed in accordance with the information signal on the bottom of 
a spiral groove in the surface of the recording medium, and, in the 
reproducing system, the reproducing stylus traces the track in a state 
wherein its stylus tip is within the spiral groove thereby to reproduce 
the information signal as variation of capacitance. 
This known system, however, has had a drawback in that special reproducing 
modes of operation such as the so-called still reproduction, wherein a 
stopped picture is obtained by reproducing the recorded information of one 
revolution of the recording medium by repetition thereof through a 
plurality of revolutions, cannot be carried out. The reason for this is 
that, since the reproducing stylus is placed within the spiral groove in 
the recording medium, it would be forced to ride over the groove wall if 
an attempt were to be made to carry out this still reproduction. This 
would result in skipping of the stylus and also damage to the stylus and 
the groove wall of the recording medium. This case is true also in the 
slow motion picture and quick motion picture reproductions. 
Another problem which has arisen in the prior art is due to the necessity 
of recording a video signal with high density on the recording medium 
since the information frequency band of a video signal, in general, is as 
large as 200 times that of an audio signal. For this reason, the width of 
the groove in the recording medium described above is made very narrow, 
being less than a number of .mu.m. Moreover, since the reproducing stylus 
is continually in sliding contact with only one groove, the area of the 
stylus tip in contact with the groove is small, whereby the contact 
pressure acting on the stylus tip per unit area thereof is extremely great 
such as 10 mg/.mu.m.sup.2, for example. For this reason, the wear of the 
reproducing stylus and the recording medium is remarkably rapid and gives 
rise to drawbacks such as short serviceable life. 
If the stylus contact pressure is decreased with the aim of reducing this 
wear, so-called stylus skipping will occur, and stable reproduction cannot 
be achieved. Furthermore, a decrease in the stylus contact force will give 
rise to problems such as an increase in the effect of dust and other 
foreign matter adhering to the recording medium. 
Accordingly, we have previously proposed a system wherein, in the recording 
system thereof, pits are formed in accordance with the information signal 
being recorded along a spiral track on a recording medium of flat disc 
shape, without forming a groove therein, and, in the reproducing system, a 
reproducing stylus traces over and along this track thereby to reproduce 
the recorded information signal. This system is disclosed in the 
specification of our U.S. patent application Ser. No. 785,095 filed Apr. 
6, 1977, entitled "Information Signal Recording and Reproducing System". 
In this proposed system, pilot signals are recorded on or in the vicinity 
of a track of the information signal such as video signal on a rotary 
disc. At the time of reproducing, the pilot signals are reproduced 
together with the video signal, and tracking servo control is carried out 
so that the reproducing stylus traces accurately along the track in 
response to the reproduced pilot signals. 
By the use of this previously proposed system, since the recording track 
has no groove, there is no possibility whatsoever of the reproducing 
stylus or the recording medium being damaged, and the stylus can trace the 
same portion of the track repeatedly a plurality of times, whereby a 
special reproduction such as still, slow motion, or quick motion 
reproduction becomes possible. Furthermore, while it is necessary for the 
electrode of the reproducing stylus to trace along only one track, a 
sliding contact surface of the main structure of the reproducing stylus 
for contacting with the recording medium can be made flat and with a wide 
width without being restricted by the width dimension of the recorded 
track, since the recorded track has no groove. For this reason, the rates 
of wear of the reproducing stylus and the recording medium are low, and 
the serviceable lives thereof are prolonged. 
A conventional reproducing stylus used in systems such as the above 
described system previously proposed has the shape of a truncated 
triangular pyramid, an electrode being provided on one of its faces. For 
this reason, as this known reproducing stylus becomes worn with use, the 
width of the part thereof with the electrode becomes progressively large 
in proportion to the wear. Then, when this width of the electrode part 
becomes greater than the track pitch interval between adjacent track of 
the recording medium, the electrode part of the stylus begins to trace and 
reproduce simultaneously two adjacent tracks. As a consequence, beats 
occur in the reproduced signal, and the stylus becomes worthless for 
practical use. Accordingly, when the width of the electrode part becomes 
close to the track pitch interval, the life of the reproducing stylus 
ends. Therefore, while this conventional reproducing stylus has a longer 
life than the reproducing stylus of the system using a recording medium 
with a track groove, it is still accompanied by the problem of short life, 
which is of the order of a number of tens or hours, for example, in the 
case of a stylus having a main structure of sapphire. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the present invention to provide a 
novel and useful stylus for reproducing an information signal from a 
recording medium which stylus is free of the problems accompanying known 
reproducing styluses. 
Another and specific object of the invention is to provide a stylus for 
reproducing an information signal from a recording medium which stylus has 
an electrode of a shape such that its electrode width remains constant as 
its wear progresses. By this provision, long time is required for the 
width of the electrode part in a direction of width of the track to become 
greater than the track pitch interval of the recording medium even when 
the wear of the electrode part has progressed to a great extent after a 
long period of use. For this reason the life of the reproducing stylus is 
greatly prolonged. 
Still another object of the invention is to provide a stylus for 
reproducing an information signal from a recording medium which stylus is 
of a shape such that the width of its main structure increases in a 
proportion such as to become greater than the width of the electrode part 
as its wear progresses. Since the area of the contact face of the stylus 
tip increases with the progress of wear of the reproducing stylus, the 
stylus force per unit area becomes small, and the wear of the reproducing 
stylus and that of the recording medium are reduced. This also contributes 
to the prolonging of the life of the reproducing stylus. 
A further object of the invention is to provide a stylus for reproducing an 
information signal from a recording medium which stylus can be easily and 
stably ground and can be easily produced. 
Other objects and further features of the invention will be apparent from 
the following detailed description with respect to preferred embodiments 
of the invention when read in conjunction with the accompanying drawings.

DETAILED DESCRIPTION 
One example of a reproducing transducer to which a reproducing stylus of 
the invention can be applied will now be described with reference to FIG. 
1A. A cantilever 11 having at its free end a reproducing stylus 10 is 
mounted at its proximal end via a damper 12 on a bracket 13. The bracket 
13 is secured to a support plate 14 mounted on a re-entrant cylindrical 
cavity resonator (not shown). 
The reproducing stylus 10 comprises a stylus structure made of diamond or 
sapphire and an electrode consisting of an electroconductive film adhered 
onto the end face of the stylus structure, as described hereinafter. This 
electrode is connected through a very thin lead wire 15 which is flexible 
and slackened to a terminal 16 provided on the support plate 14. As the 
reproducing stylus 10 traces along a track on a rotary disc 17, the video 
signal recorded thereon as variation of pits is reproduced as variation in 
the electrostatic capacitance between the surface of the rotary disc 17 
and the electrode of the reproducing stylus 10. 
In the case where there are fluctuations of the rotating surface of the 
rotary disc 17, the reproducing stylus 10 moves in a path as viewed in 
side view which is, strictly speaking, an arc of a circle. However, since 
the amplitude of the rotating surface fluctuation is of the order of 100 
.mu.m, it may be assumed, when the length of the cantilever 11 is selected 
at a value of the order of 30 mm., for example, that the reproducing 
stylus 10 undergoes an up-and-down following displacement in a 
substantially straight line in conformance with the rotating surface 
fluctuation. 
A very thin gold wire 19 of a diameter of the order of 20 .mu.m, for 
example, is bonded, except at its ends, by an instanteneous adhesive to 
the cantilever 11 over a specific distance in the longitudinal direction 
thereof. Lead wire parts 19a and 19b of the end parts of this gold wire 19 
are respectively connected in a slackened state to terminals 20a and 20b 
provided on the support plate 14. 
A permanent magnet 21 is secured by way of a bracket 22 to the lower 
surface of the support plate 14. The wire 19 is thus disposed in this 
strong magnetic field formed by the magnet 21. 
The rotary disc 17 has a plurality of pits 18 formed therein along a spiral 
track in accordance with the recorded video signal as shown in FIG. 1B. 
This track does not have a groove for guiding the reproducing stylus but 
consists merely of the pits 18 formed in the flat surface of the disc. The 
spiral track has successive track t.sub.1, t.sub.2, t.sub.3, . . . 
arranged side-by-side, each being the track portion corresponding to one 
revolution of the rotary disc 17. At and along the lateral edges of each 
track, pits 23 and 24 of pilot signals of mutually different frequencies 
fp.sub.1 and fp.sub.2 are respectively formed in alternate staggered 
formation in the disc. One half of each of these pits 23 and 24 overlaps 
the end portions of a certain number of the pits 18 of the track, while 
the other half of the same pit overlaps the end portions of a certain 
number of the pits 18 of the contiguously adjacent track. 
In the instant example, the rotary disc, which has a 12-inch diameter, is 
rotated at a speed of 900 rpm., and has a capacity of recording an 
information signal of approximately 30 minutes. The tracks are formed 
contiguously in side-by-side arrangement, and the track pitch is equal to 
the track width, being 2.6 .mu.m., for example. Furthermore, the 
frequencies fp.sub.1 and fp.sub.2 of the pilot signals are respectively 
511 KHz and 716 KHz, and the track width of the pilot signals is 0.6 
.mu.m. The rotary disc 17 comprises, for example, a disc substrate made of 
polyvinyl acetate (PVAC), in which the pits 18 are formed along the spiral 
track, a metal coating formed with a thickness of 400 A, for example, on 
the disc substrate to constitute an electrode, and a dielectric coating 
film of a thickness of 400 A adhered onto the metal coating. The disc may 
be caused to possess an electrode function by using another material 
instead of providing a metal coating film. 
At the time of reproduction, the rotary disc 17 rotates, and the 
reproducing stylus 10 traces relatively the tracks t.sub.1, t.sub.2, . . . 
, whereupon the recorded video signal and the pilot signals fp.sub.1 and 
fp.sub.2 are reproduced through the reproducing stylus 10. The reproduced 
signal thus picked up is supplied to a control signal generating circuit. 
A control signal is derived from the control signal circuit in accordance 
with the deviation of the reproducing stylus 10 from each track and 
supplied to terminals 20a and 20b. 
When a control signal current flows through the terminals 20a and 20b and 
through the wire 19, a displacement force acts on the wire 19 in the 
transverse direction perpendicular to the longitudinal direction of the 
cantilever 11 in response to the above mentioned current, according to the 
Fleming's left-hand rule, since the wire 19 is disposed within the 
magnetic field due to the magnet 21. Since the wire 19 is bonded to the 
cantilever 11, the cantilever 11 is displaced by the force acting on the 
wire 19. As a result, the reproducing stylus 10 is displaced in a 
direction perpendicular to the longitudinal direction of the track, and 
tracking control is so accomplished that the reproducing stylus 10 traces 
accurately along the track. 
For the reproducing stylus 10, a reproducing stylus 30 of the shape shown 
in FIG. 2 has heretofore been used. This reproducing stylus 30 has a 
stylus main structure 31 made of a wear resistant material such as diamond 
or sapphire and having the shape of a triangular pyramid with three faces 
forming edge lines 32a, 32b, and 32c. The vertex part (which is the bottom 
part when the stylus is being used) is truncated and thereby has a contact 
face 34 for sliding on the rotary disc 17. One of the above mentioned 
faces is provided with an electrode 35 of a conductor of high 
electroconductivity such as, for example, hafnium or titanium, formed or 
adhered by sputtering. 
This electrode 35 has the shape of a truncated triangle defined by the edge 
lines 32a and 32b forming an angle .theta. therebetween. That is, this 
electrode 35 possesses a shape which is the same as the shape which 
results when the reproducing stylus main structure 31 is projected onto a 
plane parallel to the electrode 35. The tip angle .theta. of the stylus 
main structure 31 is selected at approximately 40 degrees, since if it is 
made too small, the stylus main structure 31 will chip or break. For this 
reason, when the reproducing stylus 30 is used with its sliding contact 
face 34 in contact with the rotary disc 17, and as the stylus tip becomes 
progressively worn, the width of the electrode 35 in the direction of the 
width of the track of the rotary disc at the sliding contact face 34 
increases in proportion to the magnitude of the wear. For example, during 
the initial period of use, the width of the electrode 35 at the sliding 
contact face 34 is P1, but, as the wear progresses as indicated by the 
sliding contact face 34a shown by broken line, the width of the electrode 
35 becomes large as indicated at P2 in FIG. 2. 
When the width P2 of this electrode 35 becomes greater than the dimension 
of the track pitch TP of the tracks t.sub.1, t.sub.2, t.sub.3, . . . on 
the rotary disc 17, the adjacent tracks are simultaneously reproduced. For 
this reason, the reproducing stylus becomes useless, and its serviceable 
life terminates. Consequently, the life of this known reproducing stylus 
has been relatively short. 
The manner in which the difficulties associated with the above described 
known reproducing stylus are overcome in the reproducing stylus according 
to the present invention will be apparent from the following description 
with respect to preferred embodiments of the invention. In this 
specification and the appended claims, the widths of the electrode and the 
slide contact face of the reproducing stylus main structure are their 
widths in the width direction of the track of the rotary disc. The length 
of the sliding contact face is its length in the extending direction of 
the track. 
In a first embodiment of the reproducing stylus according to the invention 
as shown in FIG. 3, a reproducing stylus of a shape similar to that of the 
known stylus illustrated in FIG. 2 is first obtained in the fabrication of 
a reproducing stylus 40. First, an electrode 42 of a film thickness of the 
order of 1,000 A to 2,000 A, for example, is formed by applying a film of 
an electroconductor such as hafnium or titanium by sputtering on one face 
of a stylus main structure 41 formed from diamond or sapphire. Next, the 
stylus main structure is so ground that it acquires two other faces which, 
with the face of the electrode 42, form edge lines 43a and 43b and 
mutually form an edge line 44. In addition, the vertex part of the main 
structure 41 is so ground that the width of the electrode 42 becomes a 
prescribed width P thereby to form the sliding contact face 45. Thus a 
stylus main structure of a shape substantially similar to that shown in 
FIG. 2 is obtained. 
Then the parts of this stylus main structure 41 at the upper ends of the 
edge lines 43a and 43b are ground off, together with the corresponding 
parts of the electrode 42, in a manner such that the width of the 
electrode 42 in the vicinity of the sliding contact face 45 is constant at 
the above mentioned value P over a predetermined distance h (for example, 
several .mu.m or more) extending downward from the sliding contact face 
45, thereby forming cutout recesses 46a and 46b. Thus, the reproducing 
stylus 40 of the first embodiment of the invention is obtained. 
The sliding contact face 45 (which becomes the bottom face during use) is 
flat including the foremost tip of the electrode 42 and, at the initial 
period of use of the reproducing stylus 40, is of triangular shape as 
shown by the sliding contact face 45a in FIG. 4A. When the stylus 40 is 
used for a long time, the electrode 42 and the stylus main structure at 
its tip become worn, and the sliding contact face assumes a pentagonal 
shape 45b as shown in FIG. 4B. However, even in this worn state of the 
stylus, the width of the electrode 42 is still preserved at the constant 
value P since the width of the stylus tip, together with the width of the 
electrode 42, is of the constant value P over the distance h as described 
above. Therefore, even when the stylus tip becomes considerably worn after 
a long period of use, the width of the electrode 42 does not become 
greater than the track width, whereby the life of the reproducing stylus 
is prolonged. 
Furthermore, as the stylus tip becomes worn, the width and length of the 
sliding contact face 45 respective increase from Wa to Wb and from La to 
Lb as will be apparent from a comparison of FIGS. 4A and 4B, whereby the 
area of the sliding contact face increases. As a result, the stylus 
contact pressure per unit area of contact is greately reduced to 1 
mg/.mu.m.sup.2, for example, and this reduction of the stylus contact 
pressure also contributes to the prolonging of the life of the reproducing 
stylus and the life of the rotary disc. 
However, if the length Lb of the worn sliding contact face 45b becomes 
excessively long, it will give rise to a phenomenon wherein the tip of the 
electrode 42 separates from the rotary disc 17 as the reproducing stylus 
40 undergoes up-and-down movement (strictly speaking an arcuate 
up-and-down movement) in conformance with the surface undulations of the 
rotary disc 17. Consequently stable and accurate reproduction can no 
longer be carried out. Accordingly, this phenomenon also determines a 
practical limit to the life of the reproducing stylus, and the life ends 
when this length Lb exceeds 20 .mu.m., for example. 
The tracing state of the stylus tip in its worn condition relative to the 
track on the rotary disc 17 up to the time when the sliding contact face 
45 thereof assumes the shape shown in FIG. 4B is indicated in FIG. 5. In 
the instant embodiment of the invention, the track pitch TP is, for 
example, 2.6 .mu.m., and the electrode width P is selected at 1.5 to 2.0 
.mu.m. In this connection, the part of the worn sliding contact face 45b 
constituted by the main structure 41 of the reproducing stylus is 
straddling over a plurality of track turns, but since the electrode 
occupies only the part designated by reference numeral 42, there is no 
detrimental effect due to this state of the stylus main structure 41. On 
the contrary, it affords the above described beneficial effect. 
In a second embodiment of the invention as illustrated in FIG. 6, the 
reproducing stylus 50 has an electrode 42 which is caused to have a 
constant width P over a specific distance as a result of the provision of 
cutout recesses 46a and 46b similarly as in the preceding first embodiment 
of the invention. The main structure 51 of this reproducing stylus has an 
inclined face 52 in addition to the face onto which the electrode 42 is 
adhered and the two flank faces, the face 52 being on the side opposite 
the face bearing the electrode 42. The sliding contact face 53 of the 
vertex part or tip of the stylus has a rectangular shape as indicated in 
FIG. 7A at the initial period of use. When the stylus tip becomes worn 
after a long period of use, the sliding contact face assumes a shape as 
designated by 53b of increased width and length in FIG. 7B. The width of 
the electrode 42, however, remains at its contant value P. 
In the case of the preceding first embodiment of the invention, since the 
part of the stylus at the edge line 44 constitutes the leading part of the 
stylus in the direction of tracing relative to the rotary disc 17 rotating 
in the arrow direction in FIG. 5, the stylus has the capability of 
removing dust on the rotary disc as a result of the action of this edge 
line part 44. While the stylus of this second embodiment of the invention 
does not have a dust removing capability comparable to that of the first 
embodiment of the invention, the possibility of chipping of the stylus 
during the grinding fabrication process is less. Furthermore, it has the 
advantageous possibility of making the area of the sliding contact face 
greater. 
While in the second embodiment of the invention, the sliding contact face 
53 is initially a rectangle, it is also possible to use a sliding contact 
face of trapezoidal shape such as the face 61 of the reproducing stylus 60 
constituting a third embodiment of the invention as illustrated in FIG. 8. 
A fourth embodiment of the invention will now be described with respect to 
the sequence of the fabrication steps thereof. In the preceding first, 
second, and third embodiments of the invention, the work of forming the 
cutout recesses 46a and 46b is relatively difficult, and the yield of good 
quality stylii is not so good, whereby these reproducing stylii are 
accompanied by the problem of not being easily adaptable to mass 
production. In the instant fourth embodiment of the invention, the 
reproducing stylus is so adapted that all grinding steps can be carried 
out by planar grinding thereby to overcome the above mentioned problem. 
First, as indicated in FIG. 9A, a stylus blank 70 of diamond or sapphire in 
the shape of a bar with a square cross section, each side dimension of 
which is of the order of 0.2 mm., is prepared. One lateral face 71 of this 
stylus blank 70 is ground to a mirror finish, and then a sloped first face 
72 with an inclination angle .alpha. relative to the vertical direction is 
formed by grinding to a mirror finish over a specific vertical distance h 
from the end face (upper face as viewed in FIG. 9A) of the stylus blank. 
Next, as indicated in FIG. 9B, hafnium is deposited by sputtering to form 
an electrode 73 on at least a second and the first faces 71 and 72. This 
electrode may be formed over the entire stylus blank having the sloped 
face 72 since the unnecessary parts of the stylus blank 70 are cut away in 
a subsequent process step. Parts 74a and 74b bounded and indicated by 
broken lines of the stylus blank in the state indicated in FIG. 9B are 
then removed by planar grinding, whereupon a reproducing stylus 76 of the 
shape shown in FIG. 9C is obtained. This removal of the parts 74a and 74b 
is so carried out that the width of the remaining electrode part 73a on 
the face 72 will be of a constant value P determined to be of proper 
relation with the aforementioned track pitch, and that the remaining 
electrode part 73b on the face 71 will be of trapezoidal shape with a 
vertex angle .theta. between the lateral sides of the order of 40 degrees. 
As a result, two new faces 75a and 75b are formed. 
From the reproducing stylus 76 thus formed, parts 77a and 77b bounded and 
indicated by broken lines in FIG. 9C. are then removed by planar grinding, 
whereupon a reproducing stylus 78 of the shape shown in FIG. 9D is 
obtained. This reproducing stylus 78 has an edge line 79 between the 
lateral faces resulting from the removal of the parts 77a and 77b and a 
sliding contact face 80, which is of triangular shape 80a as shown in FIG. 
10A at the initial period of use. As the stylus tip becomes worn, the 
width of the electrode part 73a continues to be of the constant value P, 
while both the width and length of the sliding contact face 80 increase as 
indicated by the face 80b in FIG. 10B. 
A reproducing stylus 81 as illustrated in FIG. 11, which is a modification 
of the reproducing stylus 76 shown in FIG. 9D, has a sliding contact face 
82 which, throughout its life from the initial period of use, has a shape 
as shown. In FIG. 11, those parts which are the same as or equivalent to 
corresponding parts in FIG. 9 are designated by like reference numerals, 
and description of such parts will be omitted. 
Included within the scope of the present invention is the construction of a 
reproducing stylus whose sliding contact face has a width greater than the 
width of the electrode tip portion after use of the stylus, even though 
the width of the sliding contact face is equal to that of the electrode 
tip portion at the time of commencing of use. 
Further, this invention is not limited to these embodiments but various 
variations and modifications may be made without departing from the scope 
and spirit of the invention.