Video disc player having lifter driving circuit

The stylus lifter driving circuit gradually lowers the stylus arm until the stylus contacts a turntable-disposed record. Once the stylus/record engagement is detected, the lifter support bracket rapidly moves away from the stylus arm to prevent stylus bouncing.

The present invention relates generally to a video disc record player and, 
more particularly, to a stylus lifter mechanism for use therewith. 
In a capacitance-type video disc system, audio and video information is 
recorded as geometric variations in a spiral information track on a disc 
record having a conductive property. A variable capacitance is formed 
between a stylus-mounted electrode and the conductive property of the disc 
record. As the record is rotated, the signal recovered at the stylus 
electrode is related to the varying capacitance, which, in turn, is 
directly related to the signal recorded in the information track. The 
recovered signal is then processed for application to a television 
receiver for playback of the recorded video and audio information. One 
example of this type of video disc system is described in U.S. Pat. No. 
3,842,194 granted to J. K. Clemens. 
In video disc systems, the information track density is generally quite 
high. For example, the above-mentioned capacitance-type system uses a 
spirally-grooved information track with a groove density of nearly 10,000 
grooves per inch (groove width=approximately 2.5 micrometers). During 
playback, the stylus tip is received in the groove and guided toward the 
record center as a turntable-supported record revolves at the desired 
speed (e.g., 450 rpm). The microscopic groove geometries require a stylus 
that has rather fine dimensions (e.g., tip width--2 micrometers). 
The pickup stylus is typically housed in a protective cartridge. The stylus 
is disposed at one end of a stylus arm having its other end suspended in 
the cartridge housing by a rubber suspension. The stylus arm is held 
inside the cartridge housing in its stored position by a pair of retaining 
fingers. A leaf spring (or a flylead) has one end secured to the stylus, 
and its other end to the cartridge housing. The flylead provides the 
stylus/record engagement force, and also serves as the electrical 
connection between the stylus electrode and the signal processing circuits 
of a video disc player. 
The stylus cartridge is inserted into a carriage translatably mounted in 
the video disc player. When the cartridge is thus installed in the 
carriage, the stylus arm retaining fingers are automatically spread apart 
to release the stylus arm. Housed in the carriage is a stylus 
lifting/lowering mechanism (also referred to as the stylus lifter 
mechanism) for selectively lowering the stylus onto a turntable-supported 
record. During playback, the carriage follows the radially inward motion 
of the groove-guided stylus. 
A concern with the stylus lifter mechanism has been the potential damage to 
the disc when the stylus is let down onto the disc and lifted up 
therefrom. High density information systems of this nature require gentle 
lowering and lifting of the stylus in order to minimize the stylus/disc 
damage. U.S. Pat. No. 4,266,785 (Burrus) describes an electromagnetic 
stylus lifter mechanism which smoothly and reliably performs the lifting 
and lowering operations. 
Basically, the Burrus lifter mechanism comprises a movably-mounted stylus 
arm support bracket, and a selectively-actuated electromagnet. The lifter 
electromagnet comprises an electrical winding defining a central volume 
which forms an air gap, and a permanent magnet attached to the stylus arm 
support bracket with its end extending into the central air gap. When 
energized, the electrical winding repels the permanent magnet out of the 
air gap to lower the stylus onto the record. When the voltage across the 
lifter winding is removed, the stylus arm support bracket automatically 
rises to lift the stylus away from the record. An advantage of this 
arrangement is that the stylus is up in the cartridge when the power is 
off. 
To ensure smooth landing and lift-off, the voltage across the lifter 
electrical winding is gradually increased and decreased. The gradual 
increase in the solenoid force can, however, have damaging side effects. 
As the stylus lands on the disc, the downward motion of the stylus arm 
support bracket is temporarily arrested until the repulsion force on the 
permanent magnet builds up sufficiently to compensate for the loss of 
downward force exerted by the stylus arm on the support bracket. The 
downward stylus arm force stems from two sources--the weight of the stylus 
arm assembly, and the stylus leaf spring force. During this interim phase, 
the stylus is only partially supported by the disc, and may bounce 
back-and-forth between the disc and the support bracket as the portion of 
the disc beneath the stylus moves toward and away from the stylus due to 
disc surface defects, warp, etc. The stylus bouncing can damage both the 
disc and stylus, and can also cause stylus mistracking. 
The stylus lifter driving circuit, in accordance with this invention, 
overcomes the above-mentioned problems. The lifter driving circuit 
comprises a first energizing means for gradually increasing the lifter 
force to gently move the stylus toward the disc until the stylus/record 
engagement is established. The lifter driving circuit further includes a 
second energizing means for rapidly increasing the lifter force to cause 
the stylus arm support bracket to quickly clear the stylus arm when the 
stylus/record contact is detected.

Shown in FIG. 1 is a pickup cartridge 10 suitable for use with the stylus 
lifter mechanism of the present invention. A pickup stylus 12 is disposed 
at one end of a stylus arm 14, with its other end suspended in the 
cartridge housing 16 by a compliant rubber suspension 18. A leaf spring 
20, also referred to as the flylead, is connected between the stylus 12 
and a metallic rivet 22 on the cartridge housing 16. The stylus flylead 20 
provides an electrical connection between a stylus-mounted electrode 24 
and the player pickup circuitry in the manner described later. The stylus 
flylead 20 additionally serves to provide stylus/record tracking force 
(e.g., 60 to 70 milligrams) during playback. The cartridge 10 is fitted 
with a pair of retaining fingers 26 and 28 to hold the stylus arm 14 in a 
V-shaped groove 30. 
The cartridge 10 is installed in a carriage 50 shown in FIGS. 2 and 3. Upon 
installation, the cartridge terminal 22 contacts a terminal 52 of pickup 
circuits 54. When the carriage lid 56 is closed, a pair of depending tabs 
58 and 60 disposed thereon defeat the cartridge retaining fingers 26 and 
28 to release the stylus arm 14. The carriage 50 is fitted with a wire 
hoop 62 to lock the lid 56 shut. A leaf spring 64, attached to the 
underside of the lid 56, firmly seats the cartridge 10 in the carriage 50 
upon closure. 
Housed in the carriage 50 is the subject stylus lifter mechanism 70 
comprising a pivotally-mounted, stylus arm support bracket 72, and a 
selectively-actuated electromagnet 74. The stylus arm 14 rests on the 
lifter support bracket 72 as the cartridge retaining fingers 26 and 28 are 
spread apart upon insertion of the cartridge 10 in the carriage 50. The 
lifter electromagnet 74 includes an electrical winding 76 forming a 
central volume defining an air-gap, and a permanent magnet 78 attached to 
the stylus arm support bracket, and having a portion thereof projecting 
into the central volume of the electrical winding. The lifter electrical 
winding 76 is equipped with a terminal 80 which is connected to the 
terminal 82 of a lifter driving circuit 84 of this invention. The other 
end of the electrical winding 76 is connected to the ground potential. 
When the lifter electrical winding 76 is energized, the permanent magnet 
78 is repelled out of the central volume of the winding to gently lower 
the stylus 12 onto a rotating, turntable-supported disc 86 through an 
opening 88 in the bottom wall of the carriage housing 50. During playback, 
the carriage 50 follows the radially inward motion of the groove-guided 
stylus 12. When the voltage across the lifter winding 76 is reduced, the 
stylus arm support bracket 72 swings back up under the influence of 
gravity to raise the stylus 12. 
The subject lifter driver circuit 84 will now be described in conjunction 
with FIGS. 4 and 5. To lower the stylus 12, a transistor 90 in a player 
control microprocessor 92 is turned off to enable the voltage (V1) at its 
output terminal 94 to rise as shown in FIG. 5-a. A capacitor 96 starts 
charging through a pair of resistors 98 and 100 towards a +5 volt source 
with a time constant of about 6 seconds as illustrated in FIG. 5-b. An 
amplifier 102 amplifies the voltage at its input terminal 104, and applies 
it to the lifter solenoid 76. This causes a gradually increasing current 
to flow through the electrical winding 76 to gently lower the stylus 12 
onto a turntable-disposed record 86. 
When the stylus 12 contacts the record 86, the pickup circuits 54, coupled 
to the stylus electrode 24, start recovering the prerecorded signals from 
the record. U.S. Pat. No. 4,080,625 (Kawamot, et al.) describes 
illustrative pickup circuits of this type. The recovered signals are then 
processed by the player signal processing circuits 110 to convert them 
into a form suitable for application to a conventional television receiver 
112. Reference may be made to U.S. Pat. No. 4,097,899 (Yu) for an 
explanation of the player signal processing circuitry suitable for use 
with this invention. The signal processing circuits 110 include a video 
demodulator 114 for developing at its output terminal the picture signal 
information including luminance, chrominance and synchronizing components. 
The picture demodulator 114 includes a flip-flop circuit 116 which is set 
when the video carrier is detected at its input terminal 118 as indicated 
in FIG. 5-c. When this happens, the output terminal 120 of the flip-flop 
116 is pulled up (V5) as shown at point T2 in FIG. 5-d. When the output 
terminal 118 goes high, the video is unsquelched, and the capacitor 96 
begins to charge towards the +5 volt supply through a different set of 
resistors 98 and 122 with a faster time constant (i.e., approximately 115 
milliseconds) as can be seen in FIG. 5-b. The voltage V2 across the 
capacitor 96 is amplified and fed to the lifter solenoid 76 to cause the 
lifter bracket 72 to quickly move away from the stylus arm 14. The rapid 
movement of the stylus arm support bracket 72 upon establishment of the 
stylus/disc contact reduces the tendency of the stylus 12 to bounce up and 
down and damage the disc. The point T3 in FIG. 5-b corresponds to the 
fully-lowered position of the lifter bracket 72, which is indicated in 
phantom in FIG. 3. 
To raise the stylus 12, the transistor 90 of the player control 
microprocessor 92 is turned on to pull the output terminal 94 to the 
ground potential as shown at a point T4 in FIG. 5-a. The capacitor 96 
immediately commences to discharge with a time constant of about 49 
milliseconds through the resistor 98 as indicated in FIG. 5-b. At this 
time, the current through the solenoid 76 decreases, and the stylus 12 is 
lifted up. The low voltage at the microprocessor output terminal 94 pulls 
down the squelch line (SQ) of the video demodulator 114 through a diode 
124 to squelch the video output as indicated at the point T4 in FIG. 5-d. 
The point T5 in FIG. 5-c indicates the point at which the stylus 12 leaves 
the turntable-supported record 86. 
The lifter driving circuitry 84 is not only simple and inexpensive, but it 
causes the pickup stylus to move gradually toward a turntable-mounted 
record until the stylus/record engagement is established. Upon the 
establishment of the stylus/record contact, the stylus arm support bracket 
is rapidly moved away from the stylus arm to reduce the possibility of 
disc damage. 
TABLE OF VALUES 
Illustratively, the values of the various elements utilized in FIG. 4 are 
set forth below. 
RESISTORS-- 
98--1.5 kiloohms 
100--180 kiloohms 
122--2 kiloohms 
CAITOR-- 
96--33 microfarads