Skip/arm servo system for a video disc player

A skip/arm servo system for a video disc player comprises a device adapted to skip a signal sensing element, such as a stylus for detection of information recorded on a disc, in a direction crossing a recording track, a stylus position detector adapted to detect the relative position of the stylus to a cartridge movably mounting the stylus, a comparator comparing the detected relative position with a reference relative position to produce an error output, a cartridge driver responsive to the error output to move the cartridge so as to maintain the stylus in the reference relative position. A setting device is adapted to variably set the reference relative position in accordance with a reproduction mode selected by the operator, whereby a special mode reproduction can readily be accomplished or the stylus can be readily released from a locked groove in the disc.

CROSS-REFERENCES OF THE RELATED APPLICATIONS 
This application relates to a U.S. application Ser. No. 321,761 filed on 
Nov. 13, 1981, now U.S. Pat. No. 4,400,807, entitled "STYLUS POSITION 
DETECTING APATUS" by T. Furuichi et al. and based on Japanese patent 
application No. 55-159489 (Nov. 14, 1980) and assigned to the present 
assignee. 
BACKGROUND OF THE INVENTION 
This invention relates to a video disc player having a concentric or spiral 
recording track and more particularly to a video disc player with a skip 
mechanism and an arm servo system which can realize a stable regular mode 
playback or reproduction and a special playback in a mode such as a search 
mode, as fast forward mode or a fast reverse mode. 
In a video disc player with a spiral recording track serving as guide 
grooves, a scratched or defective groove due to faulty manufacturing or 
ensuing careless handling and use gives rise to an undesired condition 
called "locked groove". In order for a signal sensing element or a stylus 
used in a very high-density disc (VHD) system or a capacitance electronic 
disc (CED) system to escape from the locked groove, there is required a 
skip mechanism which can forcibly move the signal sensing element or 
stylus from the locked groove to a normal groove. 
To explain a conventional skip function, reference should be made to FIG. 1 
which shows a relative positional relation between a stylus tip 22A for 
disc signal detection and a grooved disc 10. The stylus is typically 
carried on a pickup arm or cantilever which is supported movably in both 
vertical and lateral directions. In FIG. 1, while the stylus postures as 
shown at solid lines (I) and (III) during regular reproduction, it 
postures as shown by a chained line (II) during its shifting from (I) 
position to (III) position, namely, during the skip operation. 
As will be understood easily, it is necessary to apply to the signal 
sensing element, a stylus horizontal drive force F.sub.V which is vertical 
to the direction of the groove and torque F.sub.t about the center of 
gravity of the stylus tip 22A during the skip operation as shown at arrows 
in FIG. 1. 
According to an experiment conducted by the present inventors, under the 
application of only the horizontal drive force F.sub.V, the stylus tip 22A 
sometimes shifts excessively across several of tens of grooves, for 
example. An excessive shifting exerts a corresponding reaction on the 
stylus tip 22A and the stylus tip 22A is forced to finally shift in a 
direction opposite to an intended direction. 
Under the application of only the torque F.sub.t, the stylus tip 22A merely 
rotates and the probability of skip shifting is decreased considerably. 
Illustrated in FIG. 2 is a conventional known technique for providing the 
skipper function. In FIG. 2, there are seen skip coils 51a and 51b, a skip 
magnet 53, a cantilever 54, a pickup stylus 55, and a video disc 56. 
The pickup stylus 55 is supported by the cantilever 54 and scans along a 
signal groove in the video disc 56 a recording signal recorded in the 
signal groove. The cantilever 54 is pivoted at its center of gravity, and 
the skip magnet 53 is fixedly secured to the cantilever 54 with its S 
pole, for example, located close to the cantilever and its N pole remote 
therefrom. 
The skip coils 51a and 51b are disposed on both sides of the skip magnet 53 
as shown in FIG. 2. The skip magnet 53, cantilever 54 and pickup stylus 55 
are offset from a center line 52 between the skip coils 51a and 51b toward 
the periphery of the video disc 56 i.e. in the forward direction of the 
spiral groove. 
Normally, while scanning and reproducing a recording signal on the video 
disc 56, the pickup stylus 55 moves toward the periphery of the video disc 
56, namely, in the forward direction 57a of the pickup stylus 55. The 
moving rate is given by equation (1): 
EQU 2.66 .mu.m.times.7.5 rps.div.20 .mu.m/sec (1) 
where 2.66 .mu.m is a track groove pitch on the video disc pursuant to the 
CED system standard and 7.5 rps corresponds to a disc revolution of 450 
rpm. 
In the event that the pickup stylus 55 faces a defective signal groove in 
the video disc 56 and is prevented from moving in the forward direction 
57a, resulting in a phenomenon called "locked groove" in which the same 
signal is reproduced repeatedly, the pickup stylus 55 is required to 
forcibly jump to another groove in the forward direction 57a of the pickup 
stylus 55. 
Conventionally, upon occurrence of the locked groove, currents are fed to 
the skip coils 51a and 51b so that one skip coil 51a opposing the skip 
magnet 53 may produce an N pole, for example, and the other skip coil 51b 
may produce an S pole as shown in FIG. 2. 
With the magnetic poles thus produced by the skip coils 51a and 51b, the N 
pole of the skip coil 51a exerts attractive force 59 on the S pole of skip 
magnet 53 close to the cantilever 54 in an obliquely upward direction and 
repulsive force 58 on the N pole of skip magnet 53 remote from the 
cantilever 54 in substantially the horizontal direction toward the S pole 
of the skip coil 51b. 
As a result, the pickup stylus 55 is slightly rotated about the pivotal 
fulcrum of the cantilever 54 and raised so as to jump to another groove in 
the forward direction, thereby escaping from the locked groove. 
Unless the pickup stylus 55 is offset from the center line 52 toward the 
periphery of the video disc in contrast to the precedence, the force 59 
for obliquely raising the S pole of skip magnet 53 close to the cantilever 
54 becomes insufficient and the skip operation for causing the pickup 
stylus to forcibly jump over the signal groove in the video disc 56 
becomes almost impossible. 
Based on the skip operation, a fast reproduction is carried out as will be 
described below. In a video disc player of the CED system, 8-field 
pictures are recorded for one revolution. When the skip operation is 
effected by picking up a vertical synchronizing signal added between 
picture fields, it is possible to obtain an 8+1=9 times fast forward 
reproduction and an 8-1=7 times fast reverse reproduction. 
The skip operation effected by picking up the vertical synchronizing signal 
is effective to suppress display of noise due to the skip operation 
(background noise) on the playback screen. 
The 9 times fast forward reproduction can be accomplished by sequential 
escape from the locked groove. But, to accomplish the 7 times fast reverse 
reproduction, the skip coils 51a and 51b must produce opposite magnetic 
poles to those of FIG. 2 and must be adjusted so as to be offset from the 
center line 52 between the skip coils 51a and 51b in a direction opposite 
to the forward direction 57a shown in FIG. 2. Such an adjustment, when 
taking the eccentricity of the video disc into consideration, is difficult 
to achieve and practically, the fast reverse reproduction is almost 
impossible. 
In the video disc player, the signal sensing element such as for example 
the stylus is supported by a cartridge such as an arm. The signal sensing 
element may also be supported in a suspension fashion as in a 
two-dimension tracker of the VHD system or a three-dimension actuator of 
the optical disc player. When the stylus is moved for reproduction, the 
relative position of the stylus to the pickup arm or cartridge must be 
held constant. To this end, an arm servo mechanism is provided which 
detects the relative position between the stylus and the cartridge and 
brings a detected value into coincidence with a preset value or a 
reference value. 
The arm servo mechanism itself is disclosed in the background of the 
invention in the above-mentioned Furuichi et al patent and will not be 
detailed herein. 
With the conventional skip mechanism and the arm servo mechanism, 
simultaneous production of both the torque F.sub.t and the horizontal 
drive force F.sub.V which is vertical to the groove was difficult to 
achieve. In addition, it was practically impossible to optimize the torque 
and the horizontal drive force in accordance with condition of the locked 
groove, in other words, it is difficult to attain the sequential movement 
of the stylus over grooves upon the special reproduction and the jump of 
the stylus over a desired number of grooves in the event of occurrence of 
the locked groove. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a video disc player which 
can control the jump of the signal sensing element over a desired number, 
singular or plural, of grooves (tracks) for steady escape from the locked 
groove to thereby assure a stable regular reproduction function and a 
stable special reproduction function as well. 
To accomplish the above object, according to this invention, the reference 
relative position between the signal sensing element and the pickup arm or 
cartridge is made variable in accordance with the reproduction mode such 
as a regular mode, four times mode, ten times mode, sixty times mode or 
reverse mode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Prior to describing preferred embodiments of the present invention, a skip 
device of the type applicable to the present invention will first be 
explained which is disclosed in Japanese Patent Application No. 56-26816 
by Takeda, one of the present inventors, filed on Feb. 27, 1981. 
Specifically, the disclosed skip device is adapted for a video disc player 
comprising a pick up stylus for scanning a video disc formed with guide 
grooves, a cantilever for supporting the pickup stylus, a skip magnet 
mounted on the cantilever and fixed thereto, and a pair of skip coils 
spaced apart from each other on both sides of the skip magnet to surround 
the skip magnet. The cantilever and the skip magnet are disposed 
approximately in the middle of the paired skip coils, and the center axis 
passing through magnetic poles of the paired skip coils is substantially 
aligned with one magnetic pole of skip magnet close to the cantilever. The 
diameter of winding of each skip coil is so selected that magnetic flux 
produced by each skip coil under current conduction therethrough will 
create an obliquely downward attractive force which acts on the other 
magnetic pole of the skip magnet remote from the cantilever. With this 
type of skip device, the escape from the locked groove and the fast 
forward or reverse reproduction can readily be accomplished by changing 
the direction of passage of current through the skip coils. 
Referring now to FIGS. 3 and 4, the skip device as outlined above will be 
described in greater detail. In contrast to the skip device of FIG. 2, the 
positional relation seen from FIGS. 3 and 4 clearly shows that the center 
axis passing through skip coils 51a and 51b is substantially in 
registration with a magnetic pole of skip magnet 53 close to a cantilever 
54, and that the skip magnet 53, cantilever 54 and pickup stylus 55 are 
disposed approximately in the middle of the skip coils 51a and 51b. When 
one skip coil 51a produces an N pole and the other skip coil 51b produces 
an S pole, a relatively large force 59 is exerted on an S pole of the skip 
magnet 53 close to the cantilever 54 by the N pole of the skip coil 51a in 
a direction approximately aligned with the center axis of the magnetic 
pole of skip coil 51a. 
Concurrently therewith, a relatively small force 58 is exerted on an N pole 
of skip magnet 53 remote from the cantilever 54 by the S pole of the skip 
coil 51b in an obliquely downward direction. 
Consequently, a pickup stylus 55 is slightly rotated counterclockwise about 
the pivotal fulcrum of the cantilever 54 and raised so as to jump to 
another groove in the leading direction, thus accomplishing the escape 
from the locked groove. 
A 9-times fast forward reproduction can be carried out by sequentially 
effecting the above skip operation in synchronism with each vertical 
synchronizing signal, and a 5-times fast forward reproduction can be 
carried out by sequentially effecting the above skip operation in 
synchronism with one out of two vertical synchronizing signals. 
For a reverse reproduction, the polarity of magnetic poles of the skip 
coils 51a and 51b is inverted as compared to that of FIG. 3 and the 
direction of force acting on the S and N poles of the skip magnet 53 is 
changed as shown in FIG. 4. As a result, the pickup stylus 55 is caused to 
jump in a direction 57b opposite to the forward direction 57a shown in 
FIG. 3, thus accomplishing the reverse reproduction. 
A 7-times fast reverse reproduction can be carried out by sequentially 
effecting the above reverse skip operation in synchronism with each 
vertical synchronizing signal, and a still reproduction can be carried out 
by effecting the reverse skip operation in synchronism with one revolution 
of the video disc. 
In this example, since obliquely downward force 58 and horizontal force 59 
must act on the S and N poles of the skip magnet 53 in accordance with the 
polarity of magnetic poles of the skip coils 51a and 51b as shown in FIGS. 
3 and 4, the size of the skip coils 51a and 51b is automatically limited. 
It the skip coils 51a and 51b has an excessively small winding diameter, 
the lower magnetic pole of the skip magnet 53 will deviate from the field 
region of the skip coil when the pickup stylus 55 vertically reciprocate 
to scan on the video disc 56. 
On the other hand, if the skip coil has an excessively large winding 
diameter, force 58 will approximately equal force 59 and they will act in 
parallel but opposite directions, so that the obliquely downward force 58 
disappears which is necessary for the skipper operation about the pivotal 
fulcrum of the cantilever 54. 
According to an experiment, good results can be obtained by making the 
winding diameter of skip coils 51a and 51b approximately equal to the 
length (distance between N and S poles) of skip magnet 53 and arranging 
the skip magnet 53 such that its lower pole close to the cantilever 54 
lies on the center axis of the skip coils 51a and 51b. 
The present invention develops the skip device and the technique of the arm 
servo system disclosed in the Furuichi et al patent. The disclosure of the 
U.S. patent to Furuichi et al is incorporated by reference in the present 
application. 
The operational principle of the present invention will now be described 
briefly. 
Referring to FIGS. 5 and 6, there is shown the relative position between 
the signal sensing element 22 and the cartridge 5. Especially, FIG. 5 
shows a case where the center axes of the cartridge 5 and signal sensing 
element or stylus 22 lie on the center axis 10A of the disc groove 10. 
FIG. 6 shows a case where the center axis of the stylus 22 is offset from 
the center axis 10A of the disc groove. 
With the stylus 22 centered with the disc groove as shown in FIG. 5, 
drawing force F.sub.g acting on the stylus 22 is in parallel with the 
center axis 10A of the disc groove. In contrast, with the stylus 22 not 
centered with the disc groove, drawing force F.sub.g is decomposed into a 
centripetal component F.sub.c at the rotation center 22B and a horizontal 
drive component F.sub.v which is vertical to the groove. The offset state 
as shown in FIG. 6 takes place when the arm is stationary and the stylus 
is leading along the groove or when only the arm is offset. 
The stylus rotation center 22B is supported by a resilient support. 
Accordingly, when the stylus 22 is in condition as shown in FIG. 6, a 
recovery force F.sub.r is created to return the stylus to a free position. 
Resultant force of the horizontal drive component F.sub.v and recovery 
force F.sub.r due to resiliency constitutes a main component which 
determines the horizontal drive force F.sub.v as explained with reference 
to FIG. 1. Therefore, the horizontal drive force F.sub.v required can be 
controlled by changing the relative position between the cartridge 5 and 
the stylus 22. 
Change of the relative position between the cartridge and stylus can be 
accomplished by, for example, changing the position detection reference 
voltage value in the stylus position detecting apparatus in the Furuichi 
et al. patent previously mentioned. 
The torque F.sub.t shown in FIG. 1 can be controlled by changing the 
driving degree of the skip device as shown in FIGS. 3 and 4 in accordance 
with the degree of locked groove and the reproduction mode. Also, the 
driving degree of the skip device can be changed by controlling the supply 
of power to the skip coils previously mentioned with reference to FIGS. 3 
and 4 through the change of the amount of current passage in the skipper 
coil, or the magnitude or application time of voltage applied to the 
skipper coil. 
Turning to FIG. 7, there is illustrated a skip/arm servo system embodying 
the invention as applied to particular-mode reproduction. The system as 
shown in block form in FIG. 7 comprises an input signal generator 1 for 
generating a signal adapted for a playback mode, a skip device 2, an arm 
servo 3, a controller 4 and a cartridge or pickup arm 5 for housing the 
signal sensing element or pickup sensor 22. 
Practically, the cartridge 5 may sometimes contain the skip device 2 and 
part or entirety of the arm servo 3 but for simplicity of illustration, 
the cartridge 5 shown in FIG. 7 is exemplified to incorporate only the 
pickup sensor 22. 
The input signal generator 1 includes a mode selector 12 for selection of 
the playback mode at the will of the operator, a vertical fly-back line 
period detector 13 for discriminating a vertical fly-back pulse contained 
in the reproduction signal from the video disc, and a setting unit 14 for 
permitting the supply of output to a skip driver 21 only when outputs from 
the mode selector 12 and the vertical fly-back line period detector 13 are 
present. 
The skip device 2 includes the skip driver 21. The arm servo 3 includes a 
sensor position detector 31 as disclosed in the Furuichi et al patent, for 
example, an error generator 32 and a cartridge driver 33. The controller 4 
includes a reference position setter 41, a voltage converter 42 and a 
comparator 43. The mode selector 12 and reference position setter 41 may 
be constituted by, for example, a 4 bit microcomputer Hitachi HD 38800. 
The input signal generator 1 generates a signal in accordance with the 
playback mode such as a regular mode, fast forward mode or fast reverse 
mode and supplies the signal to the skip driver 21 and the reference 
position setter 41. The manner of reference position setting will 
specifically be described later. 
The reference position setter 41 determines the relative position of the 
sensor 22 to the cartridge 5. The relative position set by the reference 
position setter 41 is converted by the voltage converter 42 into a 
reference signal and fed to the comparator 43. 
The actual position of the sensor 22 relative to the cartridge 5, on the 
other hand, is always monitored by the sensor position detector 31 and a 
signal representative of a sensor position is fed to the comparator 43. 
The comparator 43 compares the reference signal with the detected position 
signal to produce an error which is sent to the error generator 32. 
An output signal of the error generator 32 is fed to the cartridge driver 
33 and based on this signal, the driver 33 drives the cartridge 5. Through 
the operation of the arm servo 3, the position of the sensor 22 relative 
to the cartridge 5 is always held to a value set by the reference position 
setter 41. 
In the fast forward or reverse mode, the skip driver 21 included in the 
skip device 2 is energized by the signal from the mode selector 12 
included in the input signal generator 1. As a result, the sensor 22 skips 
over one or a plurality of grooves. Concurrently, the mode selector 12 
delivers a signal corresponding to the selected operation to the reference 
position setter 41 of the controller 4, and a reference position set by 
this signal operates the arm servo 3. It should be appreciated that the 
conventional skip function is responsive to only the detection of missing 
of signals. 
The above arrangement can afford to provide an (a.multidot.m.multidot.n+1) 
times special (fast) reproduction, where 
a: the number of fields recorded during one revolution of the video disc; 
n: reciprocal of the number of fields scanned during a period between one 
skipping of sensor 22 and the subsequent skipping thereof; and 
m: the number of grooves over which one skipping of sensor 22 jumps; 
positive or negative dependent on the direction of skipping. 
For m being negative, the direction of the sensor 22 driven by the skip 
driver 21, the value of the reference position setter 41 and the attendant 
driving direction of the cartridge by the cartridge driver 33 are 
obviously required to be inverted. 
As described above, this embodiment can assure not only the regular 
reproduction but also the fast forward or reverse reproduction, the slow 
forward or reverse reproduction or the repetitions reproduction on the 
same groove (n=1, a.multidot.m.multidot.n=-1). 
In the video disc player, as will be seen from the foregoing, it is also 
necessary to consider the manner of escaping from the locked groove 
phenomenon due to defective grooves. 
FIG. 8 shows, in block form, another embodiment of the invention with a 
locked groove release function. 
A signal processor 6 is adapted to demodulate and amplify a pickup signal 
from the pickup sensor 22 which is representative of a modulated video 
signal recorded on the video disc and frame number information 
corresponding to the picture field. 
A frame number decoder 7 derives frame number information alone from the 
information delivered from the signal processor 6, detects an updated 
frame number by incrementing of number counter, for example, and processes 
the updated information into an information format which is accessible by 
a locked groove detector 11. 
Assuming that a regular forward reproduction is carried out, the frame 
number which is incremented one by one will be detected by the frame 
number decoder 7. In this case, if four picture frames, for example, are 
recorded on one recording circle, the four frames will be reproduced 
repeatedly in the event of occurrence of the locked groove phenomenon. 
Namely, the frame number will be updated abnormally. 
The locked groove detector 11 constituted by, for example, a 4 bit 
microcomputer Hitachi HD 38800 judges that the locked groove has taken 
place when the increment value of the frame number becomes minus and 
informs the input signal generator 1 of the occurrence of the locked 
groove phenomenon. In response thereto, the input signal generator 1 
supplies a skip signal to the skip driver 21. The normal skip operation is 
carried out by this skip signal and the stylus 22 is caused to forcibly 
skip over one groove, thereby escaping from the locked groove condition. 
If the escape from the locked groove is not completed by the single skip 
operation, the input signal generator 1 will further supply to the skip 
driver 21 a signal that permits skipping over 2 or 3 or more grooves. 
Accordingly, this embodiment can assure rapid escape from the locked groove 
even when the defective groove is of a large scale. 
In this case, the signal from the input signal generator to the reference 
position detector 41, which signal controls the setting of the reference 
position for the arm servo, is also set in accordance with the number of 
skipping grooves by the stylus 22 as shown in Table 1. 
Generally, for the regular reproduction, the relative position of the 
stylus 22 to the cartridge 5 is preferably such that the stylus 22 is 
centered with the cartridge as shown in FIG. 5. However, for forward 
skipping, it is preferable that the cartridge 5 lead the stylus 22 in the 
forward direction (normally toward the periphery of the video disc). 
Conversely, for reverse skipping, it is preferable that the cartridge 5 
lag with respect to the stylus 22. 
According to an experiment conducted by the present inventors, good results 
were obtained by selecting the relation between the number of skipping 
grooves and the relative position as shown in Table 1. 
TABLE 1 
______________________________________ 
Skip Distance Relative 
Direction 
Groove number (.mu.m) position 
______________________________________ 
Forward Several to 50.about.100 
Cartridge leads 
several of tens 
Forward Several of tens 
150.about.200 
" 
Reverse Several to 50.about.100 
Cartridge lags 
several of tens 
Reverse Several of tens 
150.about.200 
" 
______________________________________ 
In this case, when two opposing skip coils each having 140 turns are spaced 
apart 7 mm and they are supplied with a low DC voltage of 6 to 12 V so as 
to be driven by a pulse current, good forward and reverse skipping 
operations were attained with the following current pulse widths 
(conduction time): 
(1) For skipping over several to several of tens of grooves: 100 to 200 
msec. 
(2) For skipping over several of tens of grooves: 300 to 500 msec. 
As has been described, according to the invention, the relative position of 
the stylus to the cartridge is changeable with the playback mode so that 
optimized horizontal drive force and torque can always be applied to the 
stylus tip. 
This permits extremely smooth and steady regular reproduction and 
special-mode reproduction such as fast forward or reverse reproduction as 
well as rapid and steady escape from the locked groove condition. 
While, in the foregoing, the invention has been applied to the video disc 
player having guide grooves, the fundamental technical concept of the 
present invention wherein the relative position of the stylus to the 
cartridge is selectively set in accordance with the playback mode may be 
applicable to a video disc player of the other type which has no guide 
grooves but has a spiral or concentric recording track.