Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1 ~ BACKGROUND OF THE INVENTION
¦ The present invention generally relates to signal
pickup devices in rotary recording medium reproducing
~ apparatuses, and more particularly to a signal pickup device
~ constructed to enable a reproducing element which reproduces
recorded signals from a rotary recording medium, to finely
~ follow and trace the rotary recording medium regardless
I ~ of the surface oscillation of the rotary recording medium
introduced upon rotation of the rotary recording medium.
¦ Generally, surface oscillation is introduced when
1 a rotary recording medium (hereinafter referred to as disc)
¦ is rotated at high speed, and the signal recording surface
of the disc undergoes microscopic oscillation. This oscillation
includes low frequency components and their harmonic
- 15 1 frequency components due to the surface oscillation caused
¦ by the imperfect recording surface of the disc, that is,
A~; since the recording surface of the disc is not completely -,
;1 1 flat, and irregularities exist on the recording surface.
1 Furthermore, the above oscillation also includes mid-range
¦ frequency components due to vibration in the reproducing
l apparatus and the like, and low frequency repeated component
¦ of a signal recorded as pits formed intermittently on the
disc surface (the horizontal synchronizing signal, vertical
~ synchronizing signal, and tracking reference signal in a
~ video signal, for example) and the like. Accordingly, the
above oscillation extends in a frequency range from
approximately 15 Hz to approximately ~ kHz.
The reproducing element which relatively scans
the recording surface of the disc and reproduces the
recorded signal, is required to finely trace and scan the
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1 recording surface of the disc regardless of the surface
oscillation and vibration, and reproduce the recorded signal
from the disc with desirable S/N (signal-to-noise) ratio.
Furthermore, even when pressure is applied to the
~ reproducing element so that the reproducing element finely
; traces and scans the disc surface regardless of the surface
oscillation and vibration of the disc, the amount of the
above pressure which can be applied to the reproducing
element is limited so as not to introduce scratches in
~0 ~ either the reproducing element or the disc.
Moreover, the tracing limit of the reproducing
~: element when the reproducing element is supported and pressed
against the disc with a constant force, is determined by
a constant acceleration defined by the equivalent mass at
a tip end of a cantilever which supports the reproducing
I element, and the pressing force applied to the reproducing
element. When the acceleration of the disc oscillation
becomes larger than the above acceleration, the reproducing
element cannot perform the tracing and scanning of the
signal recording surface of the disc.
In a signal pickup apparatus comprising tracking
control means for applying a force which applies stylus
pressure on a reproducing stylus, and a force which displaces
~ the reproducing stylus in a direction perpendicular with
respect to the track longitudinal direction of the disc and
performes the tracking control, to the rear end of a
cantilever which supports the reproducing element at the
tip end thereat, a tracking shift signal obtained from the
reproduced signal is applied to tracking control means by
use of a tracking servo loop. The above operation is
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1 performed in ~rder to compensate for the distortion in the
l track and eccentricity introduced upon formation of the disc,
.. ¦ or the forwarding movement error in an apparatus which
~ subjects the reproducing element to perform a forwarding
movement in the radial direction of the disc, and the like.
Normally, when the above type of a tracking servo loop is
an open servo loop, the tracking control signal is fed back
having a gain of 7G dB, for example, with respect to the
; movement of the tip end of the cantilever, and designed so that
the response extends to sufficiently high frequencies.
The resonant frequency of the parasitic vibration due to
the displacing movement of the cantilever which is included
in this tracking servo loop, must be sufficiently higher
than the frequency fc (l.5 kHz, for example) at which the
¦ above gain becomes zero.
SUMMARY OF THE INVENTION
Accordingly, a general object of the present
1 invention is to provide a novel and useful signal pickup
~ device in a rotary recording medium reproducing apparatus
which enables a reproducing element to finely trace the
recording surface of a disc by considering the above, and
in which the above described problems have been overcome.
¦ Another and more specific object of the present
~ invention is to provide a signal pickup device in a rotary
recording medium reproducing apparatus in which a reproducing
element is mounted at one end of a plate-shaped member
of which other end is supported by the tip end of a cantilever.
I Still another object of the present invention
is to provi e a signal pickup device comprising a cantilever
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assembly having an viscoelastic plate-shaped member provided
between the cantilever and the reproducing element. According
to the apparatus of the present invention, the reproducing
element can finely follow and trace the recording surface of the
rotary recording medium regardless of the surface oscillation of
the rotary recording mediu~" and can al~o be subjected to fine
tracking control.
Other objects and further features of the present
~r,
invention will be apparent from the following description when
read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. l is an exploded perspective view of an embodiment
of a signal pickup device according to the present invention;
Fig. 2 is a plan view with a part cut away, of the
signal pickup device of Fig. 1 in an assembled state;
Fig. 3 is an enlarged perspective view of the essential
part of the signal pickup device of Fig. l with a part cut away;
Fig. 4 is a graph showing the frequency spectrum of
the disc surface oscillation, and the surface oscillation
tracing characteristic of the signal pickup device according to
the present invention; and
Figs. 5(A) through 5(D) respectively are simplified
diagrams for explaining the oscillating state of the cantilever
assembly.
D~TAILED DESCRIPTION
In Figs. l and 2, a signal pickup device 10 substan-
tially comprises a cartridge 11, a guide member 12 for
-
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1 receiving and rotating the cartridge 11, and a fixed coil
mounting member 13 mounted with a coil group. The cartridge
11 comprises a case 14, a lid 15, and a cantilever assembly
17 mounted with a reproducing stylus 16.
In the cantilever assembly 17 which constructs
an essential part of the device of the present invention,
the reproducing stylus 16 is mounted at the tip end of a
plate-shaped holder 18. The rear end of the holder 18
is mounted to the tip end of a cantilever 19 constructed
I from a pipe made of light metal such as aluminum. The
tip end of the cantilever 19 is a flat rectangular shaped
¦ pipe so that the rear end of the holder 18 can be inserted
and fixed therein, or a flat shape so that the rear end
of the holder 18 can be embeddedly fixed thereto. The part
I between the front end and the pipe-shaped part of the
cantilever 19, is of a shape which smoothly connects the ~-
shapes on both sides of the cantilever 19. The cantilever
19 is formed by forming the small-diameter end of a tapered
I pipe having circular cross-section, being seamless or
comprising seams, into a flat rectangular shape, or by forming
¦ from a thin plate and subjecting to bending and welding
¦ process.
The holder 18 is made from a flat polypropylene
~ plate, for example, in which the thickness t is 0.3 mili-meters,
~ the width b is 1.4 mili-meters, and the length Q is 5 mili-
meters. The holder 18 can be made from materials such
as nylon, polyethylene, and butyl rubber, which has both
elasticity and viscocity. Furthermore, the reproducing
stylus 16 comprises an electrode for reproducing signals
as a variation in the electrostatic capacitance respective
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1 1 of the pits which are formed on the disc 40 in accordance
with the signals, although the electrode is not shown in the
drawings. Moreover, the holder 18 must be made of insulative
~ material, since a conductive ribbon which makes conductive
~ connection with the above electrode exists at the upper
part of the holder 18.
When the length L of the can-tilever 19 and the
length of the holder 18 respectively are 30 mili-meters
and 5 mili-meters, it is desirable for the thickness and
~ width of the holder 18 to have a ratio relationship in
which, when the width b of the holder 18 is one (unity), for
example, the thickness is approximately 0.58 to 0.15.
Furthermore, the holder 18 independently has a resonance
~ frequency in the thickness direction of 2.7 kHz, and resonance
~ frequency in the width direction of 12 kHz, for example.
On the other hand, the resonance frequency in the thickness
(vertical) direction is 800 Hz to 1.5 kHz, and the resonance
frequency in the width direction is over 4 kHz for the
~ cantilever assembly 17, for example.
¦ In the present embodiment, 1.5 mili-meters of the
rear end of the holder 18, for exarnple, is inserted into
the tip end rectangular opening of the cantilever 17,
and fixedly adhered therein by an instant adhesive.
1~ Then holder 18 is mounted so that the holder is
25 ~ in a parallel plane with respect to the recording surface
of a rotary recording medium (disc) 40. A permanent magnet
20 having a cylindrical shape, is insertedly fixed to an
intermediate space part at the rear end of the cantilever 19.
~ The ring-shaped part of an elastic support member (suspension)
¦ 21 made of rubber, is insertedly connected to the rear end
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outer periphery of the cantilever 19. A pair of arms 21a and
21b unitarily extend in a direction perpendicular to the longi-
tudinal direction of the cantilever 19, which are provided at
both sides of the ring-shaped part of the support member 21.
The cantilever assembly 17 of the above described
construction is contained within the case 14, wherein the arms
21a and 21b of the support member 21 are pushed and inserted
into slots 22a and 22b of the case 14. The lid 15 is mounted
freely rotatable on the case 14, and when the signal pickup
apparatus is not used, the lid 15 is in a rotated position where
the reproducing stylus 16 is protected. On the other hand,
when the signal pickup apparatus is being used, the lid 15 is
rotated unto a position where the reproducing stylus is not
interfered.
The guide member 12 comprises grooves 23a and 23b for
receiving the cartridge 11, and is rotatably mounted on a fixed
coil mounting member 13 by a shaft 24. A coil 26 is screwed onto
the lower surface of the guide member 12.
The coil mounting member 13 is fixed to a carriage 25,
and comprises tracking control coils 27a, 27b, 28a, and 28b, and
jitter compensation coil 29 mounted thereon. The arms 21a and
21b of the elastic support member 21 in the above cantilever
assembly 17, are respectively inserted in the spaces formed
between the coils 27a and 28a, and the coils 27a and 27b. The
axes of the coils 27a, 27b, 28a and 28b are arranged in a direc-
tion perpendicular to the magnetized direction of the permanent
magnet 20. On the other hand, the axis of the coil 29 is arranged
in the same direction as the magnetized direction of the permanent
magnet 20.
In a state in which current is not applied to a
coil 26, the cantilever 19 is supported by the support member
21, and the reproducing stylus 16 is positioned at a height
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where the reproducing stylus 16 does not make contact with a
disc 40. Upon reproduction, when current is applied to the coil
26, the cantilever 19 receives a downward force, and rotates
downward while twisting the support member 21. Accordingly,
the reproducing stylus 16 is applied with a predetermined stylus
pressure, and makes contact with the disc 40.
By flowing a current having a level and direction res-
pective of the tracking error signal through the tracking control
coils 27a, 27b, 28a, and 28b, opposite magnetic polarities are
introduced at the edge surfaces of the opposing coils. Hence,
repulsive force is introduced on one hand, and on the other,
attractive force is introduced between the magnetic polarities
of the permanent magnet 20. Accordingly, the cantilever 19 is
displaced by a predetermined quantity in a direction the tracking
deviation is to be compensated, in the radial direction of the
disc 40 shown by arrow X of Fig. 3. Furthermore, when a jitter
compensation current having a level and polarity respective of
the jitter which is to be compensated, is passed through the
jitter compensation coil 29, a magnetic field is introduced
20 between the permanent magnet 20 and the coil 29. When a mag-
netic polarity which is the same as that of the permanent magnet
20 is introduced at the edge surface of the coil 29 opposing the
permanent magnet 20, repulsive force is introduced between the
coil 29 and the permanent magnet 20 and displaces the cantilevex
19 in the direction shown by an arrow Zl, to compensate for the
jitter. On the other hand, when a magnetic polarity
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1 I which is opposite to that of the permanent magnet 20 is
introduced at the edge surface of the coil 29 opposing the
permanent magnet 20, attractive force is introduced between
the coil 29 and the permanent magnet 20 and displaces the
cantilever 19 in the direction shown by an arrow Z2, to compensate
for the jitter.
In the present embodiment of the invention, the
j cantilever assembly 17 is subjected to divisional vibration
at a predetermined frequency within the above signal pickup
device, and the tracing acceleration ha-~ing a frequency
higher than that of the above predetermined frequency, to
enable tracing which is applicable upon surface oscillation
I extending to a wide range. Therefore, the surface oscillation
l~ tracing characteristic of the cantilever assembly 17 becomes
as follows. When the moment of inertia of the whole
: ~ cantilever assembly 17, and the twisting spring constant ,
of the support member 21 are respectively designated by Il ~.
and K~ , the resonance frequency fl determined by the above
two values, can be described by the following equation.
; 20 1 ~ 2~l ~ (Hz) ~ - - (1)
A frequency spectrum of the disc surface oscillation,
and the surface oscillation tracing amplitude characteristic
of the signal pickup device according to the present invention,
are shown in FIG. 4. The surface oscillation frequency of
~ the disc 40 is distributed in a frequency range shown by
a range I (15 Hz to 5 kHz, for example). The above resonance
frequency fl , for example, is approximately 25 Hz to 30 Hz.
¦ A curve II shows the surface oscillation tracing amplitude
l characteristic of the signal pickup device.
¦ At a fre~uency lower than the above resonance
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1 frequency fl , a quantity Y1 , which is the quantity displaced
¦¦ downwards from the signal recording surface of the disc 40,
in advance, by a static torque ~ of the support member
21, represents the surface oscillation tracing limit.
1 In this case, the static pressing force F against the stylus,
can be described by the following equation ~2).
' L2 . K~ (gr) ~ ~ ~ (2)
l In the above equation (2), the symbol L designates the
~ distance between the reproducing stylus 16 and the center
axis of the support member 21 in the longitudinal direction.
On the other hand, at frequencies higher than the
above resonance frequency fl , the amplitude described
by the equi-acceleration line (shown by dotted lines III
of FIG. 4) of the following equation (3) is the surface
; ~ oscillation tracing limit, where in equation (3), g designates
the gravity accelleration. ~-
~- ~ ~ g ~ ~ - (3)
l Accordingly, in the present embodiment of the
~ invention, the line connecting the tip end of the holder
¦ 18 and the connection point between the holder 18 and the
cantilever 19, tilt with respect to the force applied to
the stylus 16. As a result, resonance is introduced as shown
in FIGS. 5(A) and 5(B) at a frequency f2 which can be
described by the following equation (4), where the respective
symbols K~ , I2 ~ and Q designate the spring constant of
the holder 18, the moment of inertia of the part composed
of the cantilever 19 and the permanent magnet 20 (this moment
¦ of inertia I2 is substantially equal to the moment of inertia
I Il of the whole cantilever assembly 17), and the distance
l between the tip end of the holder 18 and the connection
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1 ll point between the holder 18 and the cantilever 19.
2 ~ 2~Q ~ (Hz) _ _ _ (4)
Thus, when the Q-factor of the above resonance
!l is designated by Q, the tracing amplitude at the resonance
1 frequency f2 is reduced to l/Q of the original tracing amplitude,
with respect to the -4OdB/dec equi-acceleration line in
the surface oscillation tracing characteristic shown by
~ the dotted lines III of FIG. 4.
¦ However, at a frequency higher than the resonance
frequency f2 ~ the tracing amplitude is equal to the
tracing amplitude Y2 in the static flexure quantity of the
holder 18, up to a resonance frequency f3 of the holder 18
in the vertical direction as shown by the arrow Y of FIG. 3.
~ The above tracing quantity Y2 can be described by the following
; ~5 ¦ equation ~5). Q2
; Y2 L2 K = K~ ~ - - (5) ~.
Furthermore, when the frequency is higher than the resonance
~ frequency f3 , an equi-acceleration line determined by the
l moment of inertia of the holder 18 (and the stylus 16) and
the above spring constant K~ . The vibration state of
the cantilever assembly in the above case, is shown in
FIGS. 5(C) and 5(D).
~ Hence, the tracing amplitude characteristic of
~ the present embodiment becomes as shown by the solid line II
of FIG. 4. By setting the spring constant K~ of the holder
18 to a predetermined value, and resonating at a resonance
frequency f2 as shown in FIGS. 5~A) and 5(B), the tracing
¦ amplitude decreases compared to that of the conventional
¦ apparatus at the neighboring frequencies of the resonance
~ frequency f2 . However, the tracing amplitude at frequencies
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1 1l higher than the resonance frequency f2 / can be increased.
Moreover, the level of the surface oscillation
frequency spectrum I gradually decreases with respect to
I the increase in the frequency, as shown in FIG. 4, and it
1 is known that when the frequency is further increased, the
level becomes a maximum value at a certain frequency. By
selecting the resonance frequency f2 f the cantilever assembly
17 at a frequency band lower than the surface oscillation
¦ frequency at the above maximum value, within the neighborhood
~ of the surface oscillation frequency which indicates the
¦ minimum surface oscillation amplitude, the cantilever assembly
17 can then finely trace according to the surface oscillation.
As described above, the cantilever 19 is subjected
to the microscopic displacement and control by the tracking
¦ control coils 27a through 28b, in the direction shown by
the arrow X of FIG. 3. It is essential that the force
applied to the base part of the cantilever 19 by the above ~-
coils 27a through 28b accurately displace the stylus 16 in
l the direction of the arrow X. In order for the above to
~ be performed, the resonance point of the cantilever assembly
17 in the direction of the arrow X of FIG. 3 (within the
: ~ horizontal plane), must be in a frequency band higher than
¦ the frequency fc f the frequency characteristic in the
tracking open servo loop at which frequency the response
becomes 0 dB. Hence, in the present embodiment, rigidity
is obtained in the vertical and the horizontal directions,
by shaping the tip end part of the cantilever 19 and the holder
18 into wide flat plate-shaped members.
¦ Accordingly, when the thickness (length in the
direction of the arrow Y) of the holder 18, and the width
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1 1 (lenght in the direction of the arrow X) at the connection
point between the holder 18 and the cantilever 19, are
respectively designated by t and b, the relationship between
the resonance frequency of the holder 18 in the Y direction
(vertical direction) which is fy (=f3) and the X direction
I (horizontal direction) which is fX cf FIG. 3, are as follows.
x ~ (b)~- - - - (6)
~ Therefore, by making the holder 18 wide, that is making b > t,
a cantilever assembly 17 having a resonance frequency f2
; of a predetermined value, and resonance frequency in the
horizontal dixection higher than the frequency fc ~ can
easily be designed and manufactured. Moreover, when the
number of rotations of the disc is 900 rpm, the frequency
c is approximately 1.5 to 2 kHz, and the frequency f2 is
~5 ~ approximately 500 to 800 Hz.
Furthermore, the resonance frequencies f2 and f3 ~.
can be selected independently from each other, and for
l example, when the weight of the holder 18 is reduced, only
the resonance frequency f3 can be shifted in the high
frequency range.
Further, this invention is not limited to these
embodiments but various variations and modifications may be
made without departing from the scope of the invention.
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