Velocity correction system for video disc player

A pickup cartridge for use in a velocity correction system includes a polymer bimorph element mechanically interposed between a cartridge housing and a pickup arm carrying a groove-riding stylus. Signals representative of cyclical deviations in the stylus/record relative velocity are applied to the polymer bimorph element to cause displacement of the groove-riding stylus in a manner that opposes cyclical deviations.

This invention generally pertains to a video disc system, and, more 
particularly, it relates to an apparatus for maintaining the stylus/record 
relative velocity substantially constant in a video disc 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 record surface includes a coating of 
conductive material which is preferably covered with a thin deposit of 
dielectric material. A pickup stylus, supported at one end of a stylus 
arm, engages the spiral groove and includes a conductive electrode which 
establishes a capacitance with the conductive coating and the dielectric 
deposit of the record. When the record is rotated, the stylus/record 
capacitance varies in response to the geometric variations in the bottom 
of the spiral groove. The capacitance variations are converted to 
electrical signal variations by suitable signal processing circuitry 
coupled to the stylus electrode. The output signal of the signal 
processing circuitry may be coupled to a conventional television receiver 
for reproduction. The other end of the stylus arm is releasably secured to 
a stylus arm carriage of the playback system. A system of the 
aforementioned type is described in detail in U.S. Pat. No. 3,842,194, 
issued to J. K. Clemens, and entitled "INFORMATION RECORDS AND 
RECORDING/PLAYBACK SYSTEMS THEREFOR". 
In such systems, it is desirable that the stylus/record relative velocity 
be kept at a substantially constant value. In the U.S. Pat. No. 3,912,283 
issued to Hammond et al., and entitled "TURNTABLE SPEED LOCK SYSTEM", an 
apparatus is disclosed for keeping the average turntable speed at a fixed 
value (e.g., 450 rpm). 
In addition to keeping the average turntable speed at a predetermined 
value, it is desirable to compensate for instantaneous cyclical deviations 
in stylus/record relative velocity to prevent "jitter" of the displayed 
television picture. Such cyclical deviations are, for example, caused by 
record eccentricity, warpage, mechanical irregularities, etc. The cyclical 
deviations in the stylus/record relative velocity can be offset by 
imparting translatory motion to the groove-riding stylus in a manner that 
opposes such cyclical deviations. A reference may be made to the U.S. Pat. 
No. 3,711,641, issued to R. C. Palmer, and entitled "VELOCITY ADJUSTING 
SYSTEM", for an example of a translatory motion imparting means (also 
referred to herein as the "armstretcher" apparatus). 
The Palmer patent discloses a circuit for sensing deviations in the 
frequency of the horizontal synchronizing pulses derived from the signal 
pickup output to develop an error correction signal which is 
representative of the cyclical deviations in the stylus/record relative 
velocity. As an alternative to the aforementioned sensing of the 
horizontal sync frequency variations to obtain control information for the 
armstretcher transducer, such information may be derived via phase 
comparison of the color synchronizing burst component of the signals 
recovered from the disc with the output of a stable reference oscillator, 
as shown in the U.S. Pat. No. 3,965,482, issued to T. W. Burrus, and 
entitled "VELOCITY CORRECTION CIRCUIT FOR VIDEO DISCS". 
The present invention relates to an advantageous transducer apparatus for 
use in velocity correction systems. Pursuant to the instant invention, a 
novel armstretcher apparatus includes a polymer bimorph transducer, 
mechanically interposed between the stylus arm and stylus arm support and 
electrically coupled to an error correction signal developing means, for 
opposing cyclical deviations in the stylus/record relative velocity. 
In accordance with a further feature of the present invention, the polymer 
bimorph transducer is mounted within a replaceable pickup cartridge.

In FIG. 1, there is shown a video disc system 20 of the type shown in the 
Clemens' patent. The video disc system includes a turntable for rotatably 
supporting a video disc 22. A turntable drive mechanism is provided for 
rotating the turntable-supported disc 22 at a predetermined speed (e.g., 
450 rpm). The recorded information is contained in the form of geometric 
variations provided in the bottom of a smooth, spiral groove disposed on 
the major surface of the disc 22. The capacitance variations between a 
conductive coating on the disc 22 and an electrode incorporated in a 
groove-riding stylus are sensed to reconstruct the recorded video 
information during playback. 
The stylus 24 is carried at the free end of a stylus arm 26. The other end 
of the stylus arm 26 is secured to a polymer bimorph transducer 28 having 
piezoelectric characteristics mounted within a replaceable pickup 
cartridge 30. As will be explained later, the desired translational motion 
of the stylus 24 will occur upon the application of appropriate electrical 
signals to the transducer 28. The pickup cartridge 30 is demountably 
received in a carriage 32, which is driven in correlation with the 
groove-riding stylus 24 during playback to reduce the tracking error and 
the tracking load on the stylus. 
The stylus electrode is coupled to a pickup circuit 34 which translates the 
stylus/disc capacitance deviations into electrical signal variations. An 
example of the pickup circuit 34 can be found in the U.S. patent 
application, Ser. No. 743,144, filed in the name of H. Kawamoto, et al., 
on Nov. 18, 1976, entitled "PICKUP CIRCUITRY FOR A VIDEO DISC PLAYER WITH 
PRINTED CIRCUIT BOARD", and now U.S. Pat. No. 4,080,625. 
The output of the pickup circuit 34 is applied to a signal processing 
circuit 36 for transforming the input thereto onto an output composite 
signal in the NTSC format for application to a conventional television 
receiver 38. Reference may be made to the U.S. Pat. No. 3,969,757, issued 
to J. G. Amery, and entitled "COLOR IMAGE SIGNAL PROCESSING CIRCUITS", for 
an illustration of the signal processing circuit. 
A velocity correction circuit 40 is coupled to the signal processing 
circuit 36 for developing an error correction signal representative of 
cyclical variations in the stylus/record relative velocity. The polymer 
bimorph transducer 28 is electrically coupled to the output of the 
velocity correction circuit 40. The polymer bimorph transducer 28, 
responsive to the error correction signal, imparts translatory, cyclical 
motion to the groove-riding stylus in a manner that opposes variations in 
the stylus/record relative velocity during playback. For example, if the 
instantaneous, stylus/record relative velocity is increasing, the 
transducer 28 advances the stylus along the record groove. The reverse 
happens if the instantaneous stylus/record relative velocity is 
decreasing. Thus, the rate and the direction of the stylus displacement 
resulting from the operation of the transducer 28 is determined by the 
error in the stylus/record relative velocity. 
The U.S. Pat. No. 3,965,482, issued to T. W. Burrus, and entitled "VELOCITY 
CORRECTION CIRCUIT FOR VIDEO DISCS", describes one form of the velocity 
correction circuit 40. As previously indicated, the Burrus system derives 
the velocity error correction signal by phase comparison of the color 
synchronizing burst component of the signals recovered from the disc with 
the output of a stable reference oscillator. In the system illustrated in 
the Burrus patent, the velocity error correction signal energizes an 
electrical winding to cause cyclical compensatory motion of the 
groove-riding stylus. Since the polymer bimorph transducer, utilized 
pursuant to this invention, is not a current actuated device, a slight 
modification of the Burrus' velocity error correction circuit would be 
required: e.g., using a high output impedance transistor driving stage to 
apply the compensating signal to the bimorph transducer. 
FIG. 2 illustrates a perspective view of the demountable pickup cartridge 
30. FIGS. 3 and 4, respectively, show the rear view and the 
cross-sectional view of the pickup cartridge 30. 
The pickup cartridge 30 comprises a cartridge housing or body 42. One end 
of the polymer bimorph element 28 is secured to the cartridge body 42. The 
pickup arm 26 is connected to the other end of the bimorph element 28 via 
an elastic coupler 44 as shown in FIG. 4. The free end of the pickup arm 
26 carries the stylus 24. The electrode incorporated in the stylus 24 is 
coupled to a terminal 46 via a conductive leaf spring 48. When the 
cartridge 30 is received in the carriage housing 32, the terminal 46 is 
connected to the pickup circuit 34. 
The cartridge 30 further includes a wire spring 50 for releasably holding 
the pickup arm 26 in the retracted condition within the confines of the 
cartridge body 42. When the cartridge 30 is installed in the player, a 
lever (not shown) enters the cartridge body through an opening 52 to 
defeat the wire spring 50, thereby releasing the pickup arm 26. 
The transducer 28 is electrically coupled to a pair of terminals 54 and 56 
shown in FIG. 3. Electrical contact is established between the output 
terminals of the velocity correction circuit 40 and the terminals 54 and 
56 of the transducer upon containment of the cartridge 30 in the carriage 
housing 32. 
FIG. 5 shows construction details of an illustrative configuration of the 
multi-layer, polymer, bimorph transducer 28. The transducer 28 comprises 
corrugated, metallized strips 58 and 60 of polyvinylidene fluoride 
membrane (PVF.sub.2) having piezoelectric properties. The adjacent folds 
of the corrugated metallized strips are bonded together by a 
non-conductive bonding agent. The polymer strips 58 and 60 are connected 
to each other by means of blocks 62 and 74. The blocks 62 and 64 can be 
formed from material such as plastic. The opposite edges of the bimorph 
element 28 are secured, respectively, to the cartridge body 42 and the 
pickup arm coupler 44. 
Illustratively, the dimensions of the transducer 28 are as follows: the 
thickness of the polyvinylidene fluoride (PVF.sub.2) film--9 micrometers, 
the thickness of the metallization on the PVF.sub.2 film--400 angstroms, 
the thickness of the bonding agent layers--0.3 micrometers, transducer 
width (dimension perpendicular to the plane of FIG. 5)--25 millimeters, 
transducer height--17 millimeters, overall transducer depth (transducer 
dimension parallel to the longitudinal axis of the pickup arm)--1 
millimeter. 
The polyvinylidene film is of the type manufactured by KUREHA Corporation 
of America, 420 Lexington Avenue, Suite 2144, New York, New York, 10017. 
The metallization is in the form of aluminum coatings. The adjacent folds 
of the corrugated PVF.sub.2 film strips are bonded together by epoxy 
resin. It is important to minimize the thickness of the epoxy layers, 
since the epoxy layers are not actively useful in the device action. 
Usually, liquid epoxy resin becomes solid after mixing with hardener. The 
epoxy resin and the hardener are separately coated on the surfaces to be 
bound, and almost completely wiped off. Thus, a very thin layer covers 
each surface and does not harden during the coating and wiping off 
process. The surfaces are then pressed together to form a tight bond. 
The design of the polymer bimorph transducer 28 (e.g., number of layers, 
dimensions, etc.) is governed by considerations such as: resonance 
characteristics of the device, peak-to-peak stylus displacement required, 
mechanical load coupled to the transducer, etc. 
FIGS. 6, 7 and 8 show alternative configurations of the multi-layer, 
polymer bimorph transducer suitable for use in the velocity correction 
system. In FIG. 6, the numerals 66 and 68 denote polymer films and 
numerals 70, 72 and 74 represent electrodes. The outside electrodes 70 and 
74 are connected to a terminal 76 and the middle electrode 72 is coupled 
to a terminal 78. The velocity error correction voltage is applied to the 
terminals 76 and 78. When the applied electric field (shown by dotted 
arrows) is in the same direction as the direction of polarization (shown 
by solid arrows), the polymer film extends. The polymer film shrinks when 
the applied electric field is in the opposite direction to the direction 
of the polarization. Illustratively, with the electric field polarity 
shown in FIG. 6, the end 80 of the FIG. 6 structure would bend downward if 
the opposite end 82 thereof were fixedly secured (due to elongation of the 
film 66 and the contraction of the film 68). If the polarity is reversed, 
the end 80 of the bimorph structure would bend upward. 
In the FIG. 7 bimorph structure, the numerals 84-94 denote polymer films, 
and the numerals 96-108 represent electrodes. The electrodes 96-108 are 
electrically connected to the terminals 110 and 112 as shown therein. 
In the FIG. 8 electro-motional device, the numerals 114 and 116 designate 
the corrugated polymer films, and the numerals 118, 120 and 122 represent 
electrodes. The electrodes 118-122 are electrically connected to the 
terminals 124 and 126 as shown therein. 
In the bimorph structures shown in FIGS. 7 and 8, the electric field 
direction corresponds to the polarization direction in the upper half 
layers, but these directions are opposite to each other in the lower half 
layers. 
The operation of the bimorph structures illustrated in FIGS. 7 and 8 is 
similar to that of the FIG. 6 bimorph structure. 
FIG. 9 is a diagram, partly in block form, of a modified velocity error 
correction system. The playback circuitry, herein, includes a horizontal 
sync separator which separates the horizontal sync pulses from the 
composite television signal recovered from the record. When the record is 
rotated at precisely the predetermined speed (e.g., 450 rpm), the 
frequency of the detected horizontal sync pulses is the same as the 
desired horizontal sync frequency (e.g., 15.734 KHz). If the turntable has 
a rotational speed somewhat slower or faster than the predetermined 
playback speed, the detected sync pulse frequency will be lower or higher, 
respectively, than the desired horizontal sync frequency. 
As shown in FIG. 9, the variations in the frequency of the horizontal sync 
pulses recovered from a record 128 are detected by a frequency 
discriminator 130 for developing an error voltage at the output thereof. 
An amplifier 132 amplifies and shapes the input error voltage and 
generates an output drive signal for application to a bimorph transducer 
134. The transducer 134, responsive to the drive signal, imparts cyclical 
translatory motion to a stylus arm 136 carrying a groove-riding stylus 138 
in a manner that opposes variations in the stylus-record relative 
velocity. A reference may be made to the U.S. Pat. No. 3,711,641, issued 
to R. C. Palmer, entitled "VELOCITY ADJUSTING SYSTEM", for an example of 
circuitry used for developing a velocity error correction signal utilizing 
horizontal sync pulses recovered from a disc record. 
FIG. 10 illustrates still another configuration of a velocity correction 
system pursuant to this invention. As shown therein, a pair of 
piezoelectric, polymer, multi-layer bimorph elements 140 and 142 are 
secured to each other along their edges in a push-pull arrangement. A 
stylus arm 144, carrying a groove-riding stylus 146, is secured to the 
bimorph element 140. The bimorph element 142 is secured to a support 147. 
A velocity error correction signal is applied to the bimorph elements 140 
and 142 via terminals 148 and 150 for causing compensating, cyclical, 
translatory motion of the groove-riding stylus 146. 
The polymer bimorph transducer apparatus, pursuant to this invention, is 
relatively inexpensive, possesses simple structure, occupies relatively 
little space, is light weight, has capacitive impedance, consumes very 
little power, provides stylus displacement linearity, and imposes very 
modest driving circuit requirements.