Note: Descriptions are shown in the official language in which they were submitted.
RCA 67,925
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1 The present invention relates to an improved
pickup arm assembly compatible with mechanical and electri-
cal requirements of a video disc playback system.
In certain video disc systems video information
is recorded by means of geometric variations in the bottom
- of a smooth spiral groove on the surface of a disc record.
The disc record surface includes a coating of conductive
material which is preferably covered with a thin deposit of
dielectric material. A signal pickup, supported at one end
, 10 of a pickup arm, engages the spiral groove and includes a
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conductive electrode which establishes a capacitance with
the conductive coating and the dielectric deposit of the
disc record. When the disc record is rotated, the elec-
trode-disc capacitance varies in response to the geometric
variations in the bottom of the spiral groove passing
beneath the signal pickup. The capacitance variations are
converted to electrical signal variations by suitable signal
processing circuitry coupled to the pickup electrode. The
output signal o the signal processing circuitry may be
coupled to a conventional television receiver for reproduc-
tion. The other end of the pickup arm is flexibly mounted
by a pickup arm support to a supporting structure of the
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playback system. A system of the aforementioned type is
described in detail in U.S. Patent No. 3,842,194, issued on
October 15, 1974 to Jon K. Clemens.
Vide~ disc systems of the aforementioned type
generally utilize disc records having groove densities in
the order of four to eight thousand groove convolutions per
inch. A typical video disc record of this type may have a
3 groove convolution spacing in the order of 3.5 microns. Th
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RCA 67,925
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1 fragile walls of relatively narrow grooves of the disc
record cannot be dependably relied upon to pull the pickup
arm assembly, around the pickup arm pivot support, acrGss
the entire recorded surface of the disc record. Also, in
video disc systems utilizing the variable capacitor concept,
it is desirable for accurate reproduction of the prerecorded
signals that the signal pickup electrode malntain a sub- ;
stantially constant attitude in the spiral groove. There-
fore, the supporting structure includes a radial feed drive
mechanism for traversing the supported end of the pickup arm
in proper time relationship with the radial motion of the
signal pickup tip engaged in the spiral groove so as to
continuously maintain the longitudinal axis of the pickup
arm substantially tangential to the spiral groove at the
point of engagement. Reference may be made to
U.S. Patent No. 3,870.835 issued to F.R. Stave on March 11,
1975, entltled "Stylus Control ApParatus For A Video ~isc Re-
cord Player" for an illustration of a suitable radial feed
drive mechanism for providing the indicated radial motion.
Further, in the above-mentioned ~ype video disc
systems it has been recognized that the relative motion
between the disc record and the signal pickup should be
maintained at a predetermined speed, and within specified
tolerance limits (e.g., 450 rpm, +0.01 percent), in order
to obtain high fidelity of reproduction of the prerecorded
~ignals. The predetermined speed and the specified toler-
ance limits are also necessary to assure that the horizontal
and vertical synchronizing information is stable and within
the lockup range of the deflection circuits of the tele-
vision receiver. Moreover, when the prerecorded information
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RCA 67,925
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1 is a color television signal with chrominance information
recorded as a modulated carrier signal, the recovered signal
must be stable and within the lockup range of the color
processing circuits o~ the playback system in order to reduce
color phase distortion.
The disc record/signal pickup relative speed may be
maintained at the predetermined speed, and within the speci-
fied tolerance limits by rendering the pickup arm support
subject to cyclical, translatory motion along the longitudinal
axis of the pickup arm in a manner that opposes deviations of
the instantaneous relative speed from the predetermined speed.
Illustratively, the means for imparting translatory motion to
the support memberlmay be of the type disclosed in the U.S.
Patent Number 3,711,641, issued to R. C. Palmer on January
16, 1973,entitled "Velocity Adjusting System".
! Additionally, in an advantageous pickup arrangement
for video disc systems of the aforementioned type, the pickup
arm is formed of conductive material, and is enclosed in a
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; conductive cage, so that the conductive pickup arm and the
; 20 surrounding conductive cage may serve respectively as an
inner and outer conductor of a transmission line. The trans-
mission line is capacity end loaded at the pickup arm
supported end by a series combination of capacitances which
; include an air dielectric capacitor and a voltage variable
capàcitor. The transmission line is also capacity end loaded
at the signal pickup end by a combination of capacitances
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which include the signal pickup electrode/disc record conduc-
tive coating variable capacitance. The transmission line and
associated capacitance form a tuned circuit with a resonant
3 frequency subject to variation as the signal pickup electrode/
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RCA 67,925
~4327~ ~
1 disc conductive coating capaci~ance varies. The tuned circuit
is excited with U~F oscillations from a fixed frequency
oscillator of the signal processing circuitry operating at a
frequency (e.g., 915 MHz) within an ISM-allocated band. As
S the resonant frequency of the tuned circuit varies, the
resultant UHF oscillation amplitude variations are detected
by the signal processing circuitry detector to recover the
prerecorded information. Reference may be made to U.S. Patent
No. 3,872,240 issued to D.~. Carlson, et al on March 18. 1975
entitled "Pickup Apparatus For Capacitive Video Disc Plavers With
Transmission Line Tuned Circuit" for a detailed discussion
o~ such a transmission line arrangement and signal processing
circuitry appropriate for use therewith.
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; Thus, in systems of the aforementioned type the
pickup arm performs a dual function: (1) it mechanicall~
supports the pickup electrode in signal transfer relation
with the disc record during playback, and (2) it forms an
electrical transmission line with the surrounding conductive
cage so that the prerecorded signals may be recovered. The
present invention is concerned with improvements in pickup
arms of the aorementioned type which may serve both mechan-
ical and electrical functions.
First, in connection with the pickup arm's
mechanical function, it is herein recognized as desirable to
reduce the angle included between (1) the disc record sur-
` face and (2) a line joining the signal pickup tip and an
effective pivot for high frequency, vertical arcuate motion
of the sighal pickup in the spiral groove, in order to
;~ reduce random forward and backward displacement of the signal
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`~ pickùp along the disc record spiral groove as a result of
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RCA 67,925
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1 spurious, high frequency, up-and-down motion of the signal
pickup in the spiral groove (e.g., due to "orange peel"
roughness of the disc groove surface etc.). The random
shifting of the signal pickup along the spiral groove results
in deleterious fluctuation of relative speed therebetween,
thereby causing flickering or jitter of the picture projected
on the television screen (which is analogous to "wow" in
audio playback systems). Pursuant to this first aspect of
the present invention, the effective pivot is desirably as
near to the disc record as practicable.
, Also, in connection with the pickup arm's mechanical
function, it is herein recognized as advantageous to dispose
the center of gravity of the pickup arm substantially in line -
with the cyclical, translatory motion of the pickup arm
lS support in order to reduce twisting moments on the pickup arm
support when the translatory motion imparting means is
activated. -
Further, in reference to the conductive pickup
arm's electrical function, it is herein recognized as desir-
able that electrical parameters (e.g., reactance) of the
transmission line remain substantially free from random vari-
ations during playback throughout the range of the trans-
latory motion of the pickup arm for accurate reproduction of
, the prerecorded signals.
Pursuant to an illustrative embodiment of the
i present invention, a pickup arm has an intermediate portion
! interconnecting a first end portion and a second end portion.
~n end o~ the pickup arm first end portion is secured to a
translatory motion imparting means of a disc record player
for varying the position of a signal pickup in relation to a
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RCA 67,925
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1 disc record spiral groove in a manner that opposes deviations
of signal pickup/spiral groove relative speed from a pre-
determined speed. During playback (1) the first and second
end portions are substantially parallel to the turntable
surface of the player, and (2) the intermediate portion
positions the second end portion near the turntable surface
relative to the first end portion. A signal pickup holder
carrying a signal pickup is pivoted to an end of the second
end portion remote from the pickup arm supported end in a
manner that normally aligns the axis of the holder and the
- second end portion. During playback the intermediate portion
positions the second end portion remote end, carrying the
signal pickup holder pivot, as near to the disc record as
practicable in order to reduce the angle included between
lS (a) the disc record surface and (b) a line joining the signal
pickup tip and the signal pickup holder pivot, thereby reduc-
ing deleterious, forward and backward displacement of the
signal pickup along the disc record spiral groove as a result
of up and down motion of the signal pickup in the spiral
grOove.
Pursuant to a further feature of the invention, the
pickup arm first end portion is (1) disposed in registry with
the translatory motion of the pickup arm support, and (2)
substantially longer than the remainder of the pickup arm,
in order to reduce twisting moments on the pickup arm support
when the translatory motion imparting means is activated.
According to a still further feature of the inven-
tion, the pickup arm first end portion is disposed substant-
ially parallel to an enclosing conductive cage during play-
back throughout the range of the translatory motion of the
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RCA 67,92S
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1 pickup arm in order to reduce random variations in the
electrical parameters of a transmission line formed by the
pickup arm and the enclosing cage.
Objects, features and advantages of this invention
will be more fully understood from the following detailed
description of the preferred embodiment, the appended claims
and the accompanying drawings, in which:
FIGURE 1 is a partially cut-away view of a video ~-
disc system incorporating an illustrative embodiment of a
pickup arm assembly according to the present invention;
FIGURE 2 is an elevation of the pickup arm assembly
of FIGURE l; and
FI~URE 3 diagrammatically illustrates horizontal
shifting of a signal pickup along a disc record spiral groove
, :.
as a result of vertical movement of the signal pickup in the
spiral groove about a pivot support.
Referring to the drawings, wherein like reference
numerals designate similar elements in the various views, in
FIGURES 1 and 2 a video disc player is shown having a turn-
table mounting plate 10. The player is suitable for use ina video disc system such as disclosed in the aforementioned
Clemens patent. A turntable 11 is rotatably mounted on the
turntable mounting plate 10. The upper surface of the turn- ~ -
table 11 is adapted to support a video disc record 12.
` 25 Video information is recorded by means of geometrical varia-
tions in the bottom of a smooth spiral groove 13 on the sub-
strate 14 of the disc record 12. The disc record 12 surface
includes a conductive coating 15 which is preferably covered
` with a thin deposit 16 of dielectric material. Although the
disc record 12 is shown to have recording on one side only
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RCA 67,925
~43Z7~
1 for the purposes of simplicity, it will be seen that the disc
record may as well have recording on both sides. A signal
pickup 17, supported at one end of a conductive pickup arm
18, engages the spiral groove 13 and includes a conductive
electrode (not shown) which, together with the conductive
coating 15 and the dielectric deposit 16, form a capacitor.
~hen relative motion is established between the signal pickup
17 and the disc record 12, an edge of the electrode included
in the signal pickup, while riding in the spiral groove 13,
serves as an electrode of a capacitor varying due to the
geometric variations in the bottom of a smooth spiral groove
passing beneath. The other end of the pickup arm l8 is
flexibly supported by a pickup arm support 20 carried by a
pickup arm supporting structure 21.
The pickup arm supporting structure 21 comprises a
radial eed drive mechanism 23 and a translatory motion
imparting means ~2. As indicated before, the radial feed
drive mechanism 23 traverses the pickup arm support 20 in
proper time relationship with the radial motion of the signal
pickup 17 tip engaged in the spiral groove 13 so as to
continuously maintain the longitudinal axis of the pickup
arm substantially tangential to the spiral groove at the
point of engagement. Reference may be made to the afore-
mentioned Stave application (U.S. Patent No. 3.~70-8~5!
for an illustration of a suitable feed drive mechanism for
providing the indicated radial motion.
~ s noted above, the translatory motion imparting
means (also known as pickup armstretcher) varies the position
; of the signal pickup 17 along the disc record spiral groove
13 by imparting cyclical, translatory motion to the pickup
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RCA 67,92~
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1 arm along its longitudinal axis, via the pickup arm support
20, in a manner that opposes deviations of instantaneous re-
lative speed from a predetermined speed. Illustratively, the
translatory motion imparting means may be of the above-
mentioned Palmer type (U.S. Patent Number 3,711,641~.
A box like, conductive cage 24 tshown with its lid
removed for clarity) is mounted to the supporting structure
21 for enclosing the conductive pickup arm 18. During play-
back, the pickup arm 18 passes through an opening 25 in the
bottom wall 26 of the housing to allow the signal pickup 17
to ride in the spiral groove 13. A pivotally mounted
bracket 27 lifts the free end of the pickup arm 18 to dis-
engage the signal pickup 17 from the spiral groove 13 when `
; the player is inoperative.
, 15 The conductive pickup arm 18 has an intermediate
; portion 30 interconnecting a first end portion 28 and a
second end portlon 29 so as to form an S-shape. An end of
th~ pickup arm first end portion 28 is yieldably secured to
the translatory motion imparting means 22 by the pickup arm
support 20. During playback (1) the first and second end
portions (28 and 29) are disposed substantially parallel to
the turntable ll surface, and (2) the intermediate portion
30 positions the second end portion 29 near the turntable
`~ surface relative to the first end portion. The pickup arm
support 20 rigidly transmits the cyclical, translatory
motion to the pickup arm 18 while accommodating vertical
and lateral, arcuate motion of the signal pickup 17 in the
spiral groove 13. A compliant signal pickup holder support
31 secures a signal pickup holder 32, carrying the signal
~i pickup 17, to an end of the pickup arm second end portion 29
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RCA 67,92~
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1 remote from the pickup arm support 20 in a manner that
normally aligns the axes of the holder and the second end
portion. The compliant signal pickup holder support 31 also
rigidly transmits the cyclical, translatory motion of the
pickup arm 18 to the signal pickup holder 32 while accommo-
dating vertical and lateral, arcutate motion of the signal
pickup 17 in the spiral groove 13.
The compliant signal pickup holder support 31,
being located adjacent to the pickup 17, reduces the pickup I -
arm assembly mass which must follow relatively high frequency,
vertical and lateral movements of the signal pickup in the
spiral groove 13. The compliant signal pickup holder support
thus accommodates relatively high frequency motion of the
signàl pickup (e.g., due to orange-peel-like rough surface
of the spiral groove), while the pickup arm sùpport 20
aacommodates relatively low frequency motion of the pickup
(e.g., due to disc mounting eccentricity, warpage, etc.).
FIGU~E 3 diagrammatically illustrates hori~ontal
displacement of a signal pickup tip 38 along a disc record
spiral groove 39 as a result of vertical movement of the
signal pickup in the spiral groove about an effective pivot
41. A line ~oining the signal pickup tip 38 and the effec-
tive pivot 41 is designated by a reference numeral 40. The
line 40 has a length dimension "R" and subtends an angle "y"
with the horizontal disc record surface 39. A horizontal
position "x" of the signal pickup tip 38 with respect to the
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pivot 41 is given by the equation x = R cos y. Therefore, `-~
horizontal displacement dx of the signal pickup tip 38 due
to vertical movement, which causes a change dy in the angle
subtended by the line 40 with the horizontal axis, is given
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RCA 67,92~
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1 by the equation ¦dx¦ = R (sin y) dy. Thus, the signal pickup
displacement dx increases with the angle y.
In the arrangement of FIGURES 1 and 2, the pickup
arm intermediate portion positions the second end portion
remote end, carrying the signal pickup holder support, as
near to the disc record surface as practicable during play-
back in order to reduce the angle subtended between (a) the
disc record surface, and (b) a line joining the tip of signal
pickup 17 tip and the pivot point provided by signal pickup
holder support 31, relative to the angle subtended between
(A) the disc record surface, and (B) a line joining the tip
of signal pickup 17 and the pivot point provided by the
pickup arm support 20. Per the relationships discussed in
connection with FIGURE 3, the consequence is a xeduction of '
; lS deleterious forward and backward displacements of the signal
pickup along the disc record spiral groove 13 that accompany
high frequency, up-and-down movements of the signal pickup
in the spiral gro~ve.
The pickup arm first end portion 28 is (1) disposed
20 in registry with ~he translatory motion imparted to the pick-
up arm support 20 by the translatory motion imparting means
22, and (2) substantially longer than the remainder of the
pickup arm, in order to dispose the center of gravity of the
pickup arm substantially in line with the translatory motion
~ 25 thereby reducing twisting moments on the pickup arm support
when the translatory motion imparting means is activated.
An end of the conductive first end portion 28
, remote fr~m the pickup arm support 20 is connected to the
signal pickup electrode by suitable means. In the illustra-
i 30 tion, respective ends of a lead are soldered to the conduc-
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RCA 67, 925
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1 tive first end portion 28 remote end and the signal pickup
17 electrode. The conductive first end portion and the
surrounding conductive cage serve respectively as an inner
and outer conductor of a transmission line. The transmission
line is capacity end loaded at both ends: (1) at the signal
pickup 17 end by a variable capacitance corresponding to the
series combination comprising (a) the varying capacitance
established between the signal pickup electrode and the disc
record conductive coating 15, and (b) the larger capacitance
10 exhibited between the bottom of the conductive cage 24 and -
the disc record conductive coating overshadowed by the cage;
and (2) at the pickup arm support 20 end by the series com-
bination including (a) an air dielectric capacitor 36, and
(b) the voltage variable capacitor (not shown). `
The air dielectric capacitor 36 includes a movable
capacitor plate 33, suspended from the pickup arm 18, ',
received in an air gap between a pair of fixed capacitor
plates (34a and 34b) fixedly mounted to the conductive cage
bottom surace 26 by an insular post 35.
The transmission line and the associated capaci-
tances establish a tuned circuit with resonant frequency
subject to variation as the signal pickup electrode/disc
record conductive coating capacitance varies. The tuned
circuit is excited with UHF oscillations from a fixed
frequency oscillator (not shown) of the signal processing
circuitry 19 enclosed in a cage compartment 37 operating at
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, a requency (e.g., 915 MHz)within an ISM-allocated band.
As the resonant frequency of the tuned circuit varies, the
resultant amplitude variations are detected by the signal
processing circuitry 19 detector to recover the prerecorded
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RCA 67,925
2~
1 information. Reference may be made to the aforementioned,
Carlson application (U.S. Patent No. 3,872,240) for
an illustration of a suitable transmission line circuit
arrangement and associated signal processing circuitry.
The conductive pickup arm first end portion 28
is disposed substantially parallel to the conductive cage
24 during playback throughout the range of the translatory
motion of the pickup arm in order to reduce random varia-
tions in the electrical parameters of the transmission line
- for accurate reproduction of the prerecorded signals.
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