Note: Descriptions are shown in the official language in which they were submitted.
~13~
BACKGROUND ~F 'FHE INVENTION
.
~ his in~ention relates to video signal recorders and,
more particularly, to a method and apparatus for cancelling the
remanent deflection which is present i~ the piezoceramic head-
~pport device of such a recorder.
Recently, piezoceramic suppori: assemblies, such as a
monomorph support leaf and a Bimorph support leaf, have been
designed to support the scanning transudcer, or hea~, in a video
signal recorder, such as a video tape recorder (VTR). ~he piezo-
ceramic support device is responsive to a control voltage appliedthereacross to ~e,flect in a direction and by an amount which is
determined by the polarity and magnitude of the control voltage
supplied thereto. Thus, during a reproducing mode of the VTR,
for example, if ~he hea~ which then is scannins a previously
lS recorded record track devlates from that txack,~he piezoceramic
support device is c~ntrolled to de~lect by a specified amount so
that the head which is supported thereby is displaced by the
same amount and, hence, is brought into proper tracking alignment
with the record track which ;t is scanning. B~ properly control-
ling the piezoceramic support device, mis-tracking errors in a
normal reproducing mode, as well as in various special reproducing '
modes, such as the stop-motion mode, the slow-mGtion mode, the
quick-motion mode, and the like, can be minimized. As a conse-
yuence of minimizing such mis-tracking errors, the video picture
25 which ultimately is reproduced is of desirable quality. An ~'
example of VTR systems in which a piezoceramic suppor~ device is
used is disclosed in Canadian Patent No. 1,106,494, Granted
August 4, 1981, and also in U.S. Patent No. 4,1~3,9940 Examples
of the material which can be used to construct the piezoceramic
support device are described in
.~................. --1-- : . .
1~3f~
Journal o~ Applied Physics, Volume 46, No. 1, January ~75,
pages 222 229; and in the paper ~Effects of Ageing and
Compressive Stress on the Properties of BaTiO3 Ceramics" by
~. J. Poole, Journal of Physics_ (Great Britai~), Volume 8, 1975.
When a piezoceramic support device, such as a bimorph
leaf assembly constructed of two piezoceramic leaves who~e direc- -
tions of polarization are opposite to each other, is supplied
with a control voltage of given polarity, the piezoceramic support
device bends, or deflects, in a direction which is determined by
that polarity. If the polarity of the control voltage is reversed,
the direction of deflection likewise is reversed. During a repro-
ducing operation, it is expected that a tracking control voltage
of changin~ magnitude and polari~y will be applied to the piezo-
ceram~c support device to correct for mis-tracking errors. It
has been found that the piezoceramic material exhibits hysteresis.
That is, as the tracking control voltage increases, a first
amount of deflection will be produced in response to a tracking
control voltage of a predetexmined level, but then, as the track-
ing control voltage is reduced, when that level of voltage once
again is reached, a second, greater (or lessex) amount of deflec- ;
tion will be obtained therefor. Because of this hysteresis, a
residual, or remanent deflection, will be present even if the
tracking control voltage is reduced to zero. This means that,
when the tracking control voltage is removed from the piezoceramic
support device, the head which is suppox~ed thereon will not return
to its "home" position.
In view of the foregoing phenomenon of remanent deflec-
tion, referred to hereinafter merely as remanence, a mis-tracking
error may result during a reproducing operation because of the
failure of the piezoceramic support device to respond to the
'
~3f~3~
tracking co~trol voltage supplied thereto in the manner for
which the tracking control system had been designed. For -
example, if the VTR is operated during an edit operation,
previously recorded video siynals first will be played back,
and then new, additional information will be recorded. Because
of the remanence of the piezoceramic !support device, the pikch
between the last track which is played back and the firsk track
which is recorded may be either too large or too small. This
discrepancy is attributed to the fact that, during the recording
section of the edit operation, no tracking control voltage is
applied to the piezoceramic support device, and the head supported
thereon is expected to return to its home position for recording.
The ai~continuity between the previously recorded and newly
recorded track~ is due to the failure of the head to return to
its home position. As another example, if the VTR is a so-called
two-head recorder, and if it is operated in the edit mode, as
before, then one piezoceramic support device may be deflected in
one direction, for example, in the upward direction, while the
other piezoceramic ~upport device is deflected in the opposite
direction in order to correct for mis-tracking errors during
the reproducing mode of the edit operation. If, after the
last track is played back, the VTR is changed over to its record-
ing mode wherein no tracking control voltage is supplied to the
piezoceramic support devices, the remanence of these devices
will result in one head being displaced upward from its home
position and the o~her head being displaced downward from its
home position. The pitch of the tracks which then are recorded
by such heads will be non-uniform, wherein the spacing between
ad~acent tracks will be greater~than-normal, followed by less-
than-normal, followed by ~reater-than-normal, and so on.
,
,
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~3~L~4~
- OBJECTS OF T~E INVENTION
Therefore, it is an object of the present invention
to provide an improved method and apparatus for cancelling the
remanence in a piezoceramic support device of the type which
is used in signal recording/reproducing apparatu~.
Another object of this invention is to provide a
method and apparatus for controlling the piezoceramic support
device of, for example, a VTR, so as to return the transducer,
or head, of that VTR to its home position in the absence of a
tracking control voltage supplied thereto.-
A further object of t~is invention is to provide a
dynamic tracking control system for use in a signal recorder/
reproducer, such as a VTR, wherein the scanning head or heads
are controlled to scan tracks of uniform pitch during both a
reproducing mode and a recording mode, regardless of the inherent
remanence of the piezoceramic head support device.
An additional object of tnis invention is to provide
improved apparatus which overcomes the aforenoted problems of
the prior art.
Various other objects, advantages and features of the
present invention will become readily apparent from the ensuing
detailed description, and the novel features will be particularly
pointed out in the appended claims.
SU~I~RY OF THE INVENTION ~ ~
In accordance with this invention, a method and apparatus ~-
are provided for cancelling the remanent deflecti`on, or remanence,
in a piezoceramic support device for a transducer wh~ch is used -
to scan successive traces across a recording medium in, for
example, a video signal recording and/or reproducing apparatus.
The apparatus is operative in selected operating modes, in one -~
_a_
~3~39L~
of which a tracking control ~ignal is supplied to th~ piezo~
ceramic 6upport device to minimize mis-tlacking of ~hR transducer.
A 60urce of oscillating ~ignal of magnitude ~ubstRntially greater
than the magnitude o the tracking control sign~l i6 provided.
When the recording and~or reproducing apparatus is changed over
between selected operating modes, the oscill~ting ~ignal is
supplied to the piezoceramic support device for a predetermined
duration. The magnitude of the oscillating ~6ignal, ~hich is
less than Ec, wherein Ec is the largest magnitude of ~he tracking
control signal to which the piezoceramic suppor~ de~ice responds,
is gradually reduced to a minimum level duriny this predetermined
duration. In one embodiment, the oscillating signal of gradually
reduced magnitude is supplied to the piezoceramic support device
when the recording and/or reproduciny appara~us is changed over
from a reproducing mode to a stand-by mode. In another embodi~
ment, the oscillating signal of gradually reduced magnitude is
supplied to the piezoceramic support device when the apparatus
is changed over to a recording mode. Preferably, ~he maximum
magnitude of the oscillating signal is within the range 0.25EC
to 0.75EC.
More particularly, there is provided:
Apparatus for recording and/or reproducing informa-
tion on a record medium in parallel skewed tr~ck6 And operative
in selected operating modes, ~aid apparatus comprising at least
-one transducer for .~canning successive traces across said record
medium; piezoceramic support means ~or supporting said at least
one transducer and responsi~e to a signal applied thereto to
deflect ~o as to correspondingly displace said transducer in a
direc_ion transversely of said scanning trace; means for upplying
a tracking control ~6ignal to said piezoceramic support means
during selected opecating modes; a source of oscillating signal
of magnïtude substantially greater than the magnitude of said
trackinq control ~ignal; and means responsive to a change-over
: .,i ~ j
-5- ~
~34~
between selected operating modes of said apparatu~ for supplying
said oscillating signal to said piezoceramic support means or
a predetermined duration duri~g which the magnitude of said
oscillating signal gradually reduces from a maximum amplitude
to a minimum amplitude.
There is also provided:
For use in a video tape recorder having a repro-
ducing mode of operation wherein vide~ ~ignals recorded in parallel
skewed tracks on magnetic tape are repr~duced, a stand-by m~de o~
operation, and/or a recording mode of operation wherein video
signals are recorded in said parallel skewed ~racks, said video
tape recorder having at least one transducer for scanning successive
traces across said tape, apparatus comprising piezoceramic :-
support means for supporting said at least one transducer and
responsive to a signal of less than a threshold magnitude Ec
applied thereto to deflect so as to correspondingly displace
said transducer in a direction transversely of said scanning trace;
means for supplying a tracking control signal to said piezoceramic
support means during said reproducing mode of operation, said
piezoceramic support means normally exhibiting a remanent deflec- .
tion when said tracking control signal is removed therefrom; a ~;
source of oscillating signal of magnitude substantially grea~er
than the magnitude of said tracking control signal but less than
Ec; and means responsive to a change-over from said reproducing ~-~
mode of operation to said stand-by mode, or to a change-over to
said recording mode of operation of said video tape recorder for
supplying said oscillating signal to said piezoceramic support
means for d predet~rmined duration, t~e ~agnitude o 6aid
oscillating signal being ~radually reduced ~rom its maximum
amplitude to a mini.mum a~plitude during at least a por~ion of :~
said predetermined duration;:whereby s~id remanent deflection
of said piezoceramic 6upport means is substantially cancelled.
~ 5a-
'` . ! ~
{3~`7
There is further provided:
A method of cancelling remanent deflection
in a piesoceramic support device for a transducer which
is used ~o scan successive traces across a recor~inq med;um
in a video signal recording and/or .reproducing apparatus, said
apparatus having a recording mode, a stand-bv mode hnd
a rproducing mode, where~n a track.ing control signal is
supplied to the piezoceramic support device during the
reproducing mode to minimize mis-tracking of the transducer;
.said met.ho~ comprising the steps of supplving an oscillating
signal to said piezoceramic support device for a predcter-
mined duration when said apparatus is changed over to its
stand-by mo~e and/or when said apparatus is changed over
to its recording mode, the magnitude of said oscillatin~
si~nal being less than Ec, wherein Ec is the largest
magnitude of the tracking control signal to which said
piezoceramic support device responds; and gradually
reducin~ the magnitude of sai~ oscillating signal to a
minimum level during said predetermined duration.
BRIEF D.SCRIPTION ~F THE DRAWINGS
.
The following detailed description, given by way of ..
exam~le, will best be understood in conjur.ction with the accom- ;
panying drawings in whic~h:
FIG. 1 is a schematic side view of a piezoc~ramic
support devioe which can be used with the pr~sent invention; .-
FIG. 2A is a side view of one embod~ment of a piezo~
ceramic head support assembly which can be used with the present
invention;
' ,'','
',''`~
' '
''',
-5b~
3~3~ 17
FIG. ~B is a top view of th~ assembly showrl in FIG. 2A;
FIG. 3 represents the relationship between the scanning
trace of a head relative to parallel record tracks on a recordi~g
medium;
FIG. 4 represents the hyste:resis of the piezoceramic
support device which is used with the present invention;
FIG. 5A is a block diagram of one embodiment of the
present invention;
FIG. 5B is a block diagram of another embodiment of
the present invention;
FIGS. 6A and 6B represent the proper, dithered tracking
of a head along a record track;
FIG. 6C represents a mis-tracking error in one direc- .
tion betweçn the head and the record track;
' FIG. 6D represents a mis-tracking error in the opposite.
direction between the head and the record track; .
FIG~ 7 is a waveform diagram of the osci`llating signal
which is used with the present invention;
FIG. 8 is a graphical representation of the relation-
ship between the remanence and the magnitude of the rernanence-
cancelling signal which is supplied to the piezoceramic support
device; and
FIG. 9 is a yraphical representation of the.relation-
ship between the responsiveness of a piezo~eramic support device
and the magnitude of the tracking control voltage supplied
thereto. `
DETAILED DESCRIPTION OF PRE.FERRED EMBODI~qENTS
Referring now to the drawings, FIG. 1 is a schematic
side view of a piezoceramic support member which can ~e used
with the present inventlon. This piezoceramic support member
~.
-6-
~1.3~
is adapted to support a transducer, such as a magnetic reoording
and/or reproducing head, and is formed of piezoceramic material
which is responsive to a voltage applied thereto so as to bend
or deflect. In the example illustrated in FIG. 1, the piezo-
ceramic support member is a so-called bimorph leaf assembly
formed of a pair of leaf members 34 and 35, each being constructed
or piezoceramic material whose direct:~ons of polarization are
represented by the illustrated arrow. The opposite surfaces of
piezoceramic leaf member 34 are plated with electrodes 34a and
34b, respectively; and the opposite surfaces of piezoceramia
leaf member 35 likewise are plated with electrodes 35a and 35b,
respectively. Piezoce~amic leaf members 34 and 35 are disposed
in overlying, face-to-face relationship such that electrodes 34b
and 35a are in contact. When assembled in this manner, the bi-
~orph assembly will deflect if a variable voltage is applied
. .
across the respective members. For example, if a voltage source40 supplies a voltage of relatively positive polarity to electrode
35b, relative to the voltage applied to electrode 35a, and if this
same positive polarity voltage is applied to electrode 34a, rela-
tive to electrode 34b, then the bimorph leaf assembly tends to
bend, or deflect, in the illustrated direction. That is, piezo- -
ceramic leaf member 34 tends to expand in its lengthwise direction
while piezoceramic leaf member 35 tends to compress. As a result
of these oppositely-acting forces, the bimorph leaf assembly
deflects as shown, and by an amount which is a function of the
strength of the electric field applied across each member. That
is, the magnitude and direction of deflection of the bimorph
leaf assembly is a function of the magnitude and polar~ty of
the voltage applied thereto by voltage source 40. If the polarity
of the voltage supplied across the bimorph leaf assembly is
.. ,
~L3~3~7
reversed, that is, if the polarity of the Yolt~ge supplied by
source 40 is opposite to that which iq illustrated, the direc-
tion in which the bimorph leaf assembly deflects correspondingly
is reversed
In FI~,. 1, a voltage is suppliad from ~ource 40 in
common with electrodes 34b and 35a. ~f the direction of polari-
zation of the piezoceramic members is made opposite to each
other, that i5, if electrode 34a of piezoceramic member 34 is
placed in contact with electrode 35a of piezoceramic member 35,
then the voltage produced by source 40 can be supplied across
the exposed electrodes of the bimorph leaf assembly, and those
electrodes which are in cnntact with each other need not be
supplied with a common, or re~erence voltage. Reference is
made to the bimorph leaf assembly which is disclosed in U.S.
e Patent No. 4,14~047.
The piezoceramic support member shown in FIG. 1 is
used as a head support device in accordance with the construc-
tion shown in FIGS. 2A and 2B, which are side and top views,
respectively, of euch a head support assembly. Although not
shown, the head support assembly of FIGS. 2~ and 2B is adapted
to be mounted on the ro~atable guide drum which generally is
provided in a video s~gnal recorder, such as a VTR, of the two-
head or of the omega-wrap configuration. The head support ~
assembly includes a mountlng base 36 having a receiving section ~ -
37 projecting upwardly therefrom, the receiving section being
adapted to receive the b~morph assembly which may be secured
thereto by a suitable adhesive. The bimorph assembly extends -
outward from the mounting base in a direction generally parallel
to a mounting plate 39, which also extends outwardly from base 36.
Preferably, tabs 40a and 40b extend upwardly from mounti`ng plate 39,
,
` ~ -8-
~3~3~
and damping members 41a and 41b are attached to these respective
tabs. Damping members 41a and 41b are provided to damp free or
resonant oscillation of the bimorph assembly wh;ch may be caused
by the forces exerted in response to the bending voltages applied
to the respective electrodes. For example, if a dither oscillat-
ing voltage is applied across the bimorph leaf assembly, damping
members 41a and 41b damp the resonant oscillation which other-
wise might occur. This damping action is achieved because the
damping members are pressed between the sides of the bimorph
assembly and tabs 40a and 40b with suitable ~orce to prevent
oscillation but not so great as to prevent deflection of the
bimorph assembly in response to the control voltage applied
thereto. As illustrated in FIGS. 2A and 2B, head 38, which is
the recording and/or reproducing transducer of the video recorder,
is-mounted to piezoceramic leaf memher 35, as by securing the
head to electrode 35b by a suitable adhesive. Respective conduct-
ing leads are connected to the corresponding electrodes of the
bimorph leaf assembly such that this ass~mbly is responsive to
a voltage supplied thereto in the manner represented by FIG. 1.
FIG. 3 illustrates record tracks which are recorded in
parallel, skewed relation on magnetic tape T. The video signals
are recorded along these tracks by, for example, head 38. As is
conventional~ each track may contain a single field interval such
that two adjacent tracks constitute a complete frame. Control
pulses CT are recorded along a longitudinal edge of tape T, each
control pulse representing the beginning of a frame, and being
located adjacent the beginning of a track. These control pulses
CT are used, during a repro~ucing operation, in a servo control
circuit for synchronizing the movement of tape T to the rotation ~-
of the playbac~ heads as such heads scan successive traces across
. ~
., - :. .
~3~34~7
the tape. Let it be assumed, fox t~e purpose of the present
discussion, that the video signal recorder is a VTR which in-
cludes two oppositely disposed heads for scanning alternate
tracks A and B across tape T. Alternatively, the V~R may be of
the so-called omega-wrap configuration containing one head. Let
it be further assumed that the VTR is operative in various operat-
ing modes. For example, the VTR may be operative in a normal
recording mode wherein the illustrated tracks and control pulses
are formed on tape T. The VTR also may be operative in a repro-
ducing mode in which the information which is recorded in theillustrated tracks is played back there~rom. This reproduced
information can be trans~itted or used in conjunction with other
broadcast material; and ultimately is used to derive a video
picture corresponding to the reproduced information. This repro
ducing mode may be a normal reproducing mode or one of various
"special" reproducing modes. As examples of the iatter are slow-
motion, stop-motion, quIck-motion, reverse-~otion, and so on.
Yet another operating mode of the YTR is the stand-by or ready -
mode wherein tape T is maintained stationary and head (or heads)
38 continues to rotate at its synchronized speed, but information
is neither recorded nor reproduced from the tape. As an example,
the stand-by mode may be assumed when the VTR is turned on, but ~ ;
neither the recording nor the reproducing mode is selected.
In general, if the same VTR is used to reproduce the
information from tape T as was used to record that information/
each scanning trace of the head (or heads) 38 during reproduction
will coincide with the scanning trace o~ that head during record-
ing. Even if there is some difference there~etween, the scanning
trace during reproduction generally will be parallel to a record
-10-
11~4~4;
track. Proper alignment than can be attained merely by adjusting
the speed at which tape T is txansported so as to bring the scan-
ning trace into coincidence with the track then being scanned.
However, if tape T had been subjected to stretching or shrinkage
subsequent to the recording of video information thereon, the
scanning trace of the head (or heads) during reproduction may
not be parallel to the record track. As a consequence thereof,
without dynamic adjustment of the scanning trace, a mis-tracking
error will be produced. A mis-tracking error also may be produced
if a different VTR is used to reproduce the video information as
was used to record that information.
In addition to the afbrenoted mis-tracking errors which
may be produced during a normal reproducing mode of operation,
the scanning trace of the reproducing head (or heads) will not
be in alignment with the record tracks during various special
reproducing modes. For example, and as shown in F~G. 3, the
head may ollow scanning trace 13 during a normal reproducing
mode, wherein tape T is transported at its normal, forward
speed. However, during, for example, a stop-motion mode wherein
tape T is maintained stationary, the head may follow scanning
trace 12 across the tape. That is, as the head scans the tape,
the beginning of its scanning trace is in proper alignment w~th
one track, and then this scanning trace traverses the ~uard band
between adjacent tracks and terminates in alignment with the
next adjacent track. Without compensating scanning trace 12,
the video picture which is reproduced during this scanning trace
will be distorted.
I~ the head (or heads) is supported on the bimorph
leaf assembly shown in, for example, FIGS. 2A and 2B, then a
control voltage wi:Ll be supplied to the bimorph assembly as the
~ .
--1 1
~3~
head scans tape T during, for example, the stop-motion mode,
so as to bring scanning trace 12 into alignment with desired
scanning trace 13. This control voltage may be a sawtoo~h
shaped waveform whose amplitude increases as t~e head conti~es
along its scanning trace. As a consequence of this control
voltage, the bimorph assembly deflects, or bends, in a direction
transverse to its scanning trace so as ~o displace the head in a
corresponding direction, thereby bringing the head into alignment
with the record track. Of course, the polarity of the control
volt3ge which is supplied to the bimorph assembly determines the
direction in which the head is laterally displaced. In the
example just described, the head is displaced in the upward
direction relative to its scanning trace As an alternative,
the bimorph leaf assembly may be de~lected in the downward direc-
tion so as to bring scanning trace 12 into alignment with, for
example, track B. In this em~odiment, the control voltage,
which also is of sawtooth shaped waveform, is of gradually
reduced amplitude as the head cont~nues along its scanning
trace.
While FIG. 3 shows the relationship between scanning
traces 12 and 13 for stop motion and normal modes of reproduction, ~ -
other analogous scanning traces may be drawn to represent other
special reproducing modes. In each of such special reproducing
mode~, the control voltage wh~ch is supplied to the ~imorph
~ assembly should be of the proper magnitude and polarity so as
to maintain the scanning trace af the head in proper alignment
with the track then being scanned. Generally, such a control
voltage is a sawtooth shaped waveform~
-12-
~3~
It has b~en found that the piezoceramic support device,
whether formed as a bimorph leaf assembly or as a monomorph leaf
assembly, exhibits h~steresis. That is, if the control voltage
which is applied across the piezoceramic support device is
increased from, for examplej a magnitude Vl to a magnitude Vz,
and passes through an intermediate magnitude VO/ the deflection
of the piezoceramic support device in response to a voltage VO
as this control voltage increases from Vl to V2 is not equal
to the deflection thereof when the voltage V~ once again is
reached as the control voltage is reduced from V2 to Vl. An
example of the hysteresis curve for a typical piezoceramic mate-
rial from which the piezoceramic support device is constructed
is illustrated in FIG. 4. The abscissa represents the control
voltage V and the ordinate represents the deflection o~ the
piezoceramic support device. As also shown, once a control
voltage is applied to the piezoceramic support device, the
device generally will not return to its so-called "home" posi-
tion of zero deflection when that control voltage is removed.
This means that if the head which is supported on this piezo-
ceramic support device is intended to exhibit zero displacementduring, for example, a recording mode, the fact that the piezo-
ceramic support device exhibits h~steresis means that, in the
absence of a tracking control voltage, which is normal during
the recording mode, a remanent or residual deflection remains
in the piezoceramic support device so as to impart some undesired
displacement to the head. As mentioned above, this remanence of
the piezoceramic support device can result in a discontinuity
between the tracks which are played back and the immediately
following tracks which are recorded during an edit operation.
This remanence also can result in non uniform pitch in the
:
~34~3~'7
successive traces of each head in a t~o-head VTR~ All this
results from the fact that the remanence of the piezoceramic
support device prevents the head from returning ~o its home
position even when a zero tracking control voltage is applied
to the support device.
The purpose of the present :invention is to remove or
cancel the remanance of the piezoceramic suppoxt device~ This
invention proceeds upon the discovery that the remanerlce can be
cancelled if the piezoceramic support device is supplied with a
voltage which follows the hysteresis curve shown in FIG. 4, but
with ever decreasing magnitude. That is, an oscillating volta~e
is supplied to the piezoceramic support device, this oscillating
voltage commencing with a relatively high magni.tude so that,
during each half cycle thereof, the piezoceramic support device
is deflected first in one direction and then in the other, and
then the magnitude of this oscillating signal gradually is
reduced so as to correspondingly reduce the amount of deflection
of the piezoceramic support device. When the osci`llating signal
finally is reduced to a minimum amplitude, such as zero, the
piezoceramic support device will be at its neutral, or home
position, and the remanence thereof will have been cancelled~ ~
One example of circuitry which is readily adapted to -
carry out the foregoing principle is illustrated in FIG. 5A.
To be complete, this illustrated circuitry also-includes the ;
tracking control circuitry which is used to supply the control
voltage to the piezoceramic support device for dynamically
adjusting the scanning trace of the head which is supported upon
that device. The illustrated circuitry is comprised of a source
of dither oscillating signal 16, an adder 17, a drive ampli~ier
1~, piezoceramic support device 19, an envelope detector 22,
-14-
. .
~34'3~'~
a phase detector 23 and a low pASS filter 24. Source 16,
which may comprlse a suitable oscillator, such as a crystal
oscillator, having an appropriate osc.illating frequency,
generates dither oscillating signal S1~, referred to hereinafter
merely as ~he dither signal. Source :L6 is connected to adder
17 which is adapted to add a d.c. volltage level to dither
signal SA. This added d.c. voltage level is an error voltage
which is produced as a function of the mis-tracking error of
the head, shown in FIG. SA as head 20, relative to the track
being scanned thereby. The output of adder 17 is referred to
as a tracking control signal and is supplied, via a change-
over switch 14, to drive amplifier 18. ~his drive amplifier
is connected to piezoceramic support device 19 and is adapted
to supply the tracking control voltage ~hereto, whereby the
piezoceramic support device deflects in a direction and by an
amount determined by this tracking control voltage.
As head 20 scans the recording medium, assumed herein
to be magnetic tape, the si~nals recorded on that medium are
reproduced thereby. Let it be assumed t~at the video signals
are recorded as frequency modulated (FM) signals. Hence, head
20 reproduces these FM video signals. The output of head 20
is supplied via a video amplifier 21 to an envelope detector 22.
The envelope detector is adapted to determine whether head 20
is in proper or mis-tracking alignment with respect to the track
being scanned thereby. That is, if the head is in proper align-
ment with the scanned track, the FM video signal reproduced by
the head exhibits a maximum envelope. However, if the scanning
trace of head 20 deviates from the record track, the envelope of
the FM video signal is reduced. The output of envelope detector
22 is connected to one input of phase detector 23, and the other
.. . . .
-15- ~
.~ .. . . .. . .....
~3~ 7
input of this phase detector i5 supplied with the dither signal
SA from source 16. Any phase differential between the detected
envelope and the dither signal is due to a mis-tracking error,
and results in an output at the phase detector. This output
is supplied through low pass filter 24 which, in turn, produces
a d.c. error voltage which represents the mis-tracking error
of head 20. This error voltage is supplied to adder 17 whereat
it is added to the dither signal SA and supplied as a tracking
control voltage to piezoceramic supp~rt device 19.
The output of video amplifier 21 also is supplied to
a demodulator and video processing circuit (not shown) from
which a video signal is derived for use either in broadcasting,
monitoring, or the llke.
The operation of the tracking control circuitry thus~
far d~scribed now will be described with reference to FIGS. 6A-6D.
FIG. 6A represents the relationship between the scanning trace
OI head 20, shown in broken lines, and the track scanned thereby.
It is appreciated that this scanning trace is oscillated, or
dithered, in response to the dither signal SA. Furthermore, it
is assumed that, in FIG. 6A, the scanning trace is in alignment
with the track. When the head is displaced to one side of the
track, as when head 20 occupies the position shown as HA, the
envelope of the FM video signal reproduced by the head is at
minimum level. This envelope is represented by the cross-hatched
area at head location HA. When the head is displaced to the
other side of the track, as represented by head position HB,
the envelope of the FM video signal reproduced by the head once
again is reduced to a minimum level, as represented by the cross-
hatched area within head position ~B. It is appreciated that,
for proper tracking of the head with respect to the track, that is,
-'6-
.
~3~7
when the scanning trace of the head is in alignment with.the
track being scanned thereby, the envelope of the reproduced
FM video signal is at the same minimum level at two positions
during each cycle of the dither signa]
S ' Let it be assumed that head 20 deviates from its
proper tracking alignment so as to be displaced from the track
as shown in FIG. 6C. Now, when head 20 occupies position HA,
the envelope of the FM video signal reproduced by head 20 i5 a
minimum, as represented by the cross hatched area at head posi-
tion HA. When the head reaches position HB, the envelope of
the FM video signal ;s less than its maximum level but, as
shown by the cross-hatched area, this envelope is fa,r greater
than the envelope which is produced at head position HA. It
is appreciated, from FIG. 6C, that the envelope o the FM video
15 signalwhich is reproduced by head'20 reaches its minimum level
only once during each cycle of the dither signal. Thus, the
frequency of the envelope waveform produced by head 20 when
the head exhibits a tracking error is one-half tKe frequency
:; of the envelope waveform for the condition that no tracking
error is present.
If the tracking error between the head and the track ~ ;
, is as shown in FIG. 6D, that is, if the head is displaced to
;~ the opposite side of the track, the waveform of the envelope
of the FM video signal reproduced by head 20 is similar to the i'~
waveform of the envelope reproduced for the condition shown in
FIG. 6C, except that the envelope waveform undergoes a phase ', ::
shift. That is, in FIG. 6D, the envelope exhibi.ts its minimum
level at head position HB. Th.is compares to the envelope pro- -
duce~ for the condition shown in FIG. 6C wherein the envelope .'~
is at its minimum level at head position HA. Hence, the envelope
-17-
~3~3~'î
which is produced for the condition shown in FIG. 6D i~ shifted
~y 180 with respect to the envelope which is produced for the
condition shown in FIG. 6C.
It may be appreciated that when head 20 is in proper
tracking alignment with the track being scanned thereby, the
envelope of the FM video signal which is reproduced by the head
is in phase with the dither signal SAo For the mis-tracking
error condition shown in FIG. 6C, the envelope of the FM video
signal reprodused by head 20 is out-of-phase with respect to
the dither signal. This out-of-phase relation may, for example,
be less than 180 and may be considered a positive phase dis-
placement. The FM video signal reproduced by head 20 for the
condition represented in FIG. 6D likewise is out-of-phase with
respect to the dither signal. This out-of-phase relation may
be greater than 180 and may be consldered a negative phase
displacement.
Returning now to FI~. 5A, envelope detector 22 produces
the envelope of the FM video signal reproducea by head 20, samples
of which are represented b, the cross-hatched areas shown for
head positions HA and HB in FIGS. 6B-6D. The phase of this
envelope with respect to the dither signal SA is detected by
phase detector 23 by using the dither signal as a detecting
signal. The amplitude of the output of phase detector 23 is
an error signal which represents the phase differential between
the detected envelope o the FM video signal and the dither
signal; and the polarity of this error signal represents the
direction (e.g. positive or negative) of this phase displacement.
Thus, it is seen that the magnitude of the error signal produced
by phase detector 23 represents the amount of the mis-tracking
error; and the polarity o this error signal represents the
-18-
~L~L,13~4 7
direction in which the mis-tracking error oacurred. That is,
ir the head is misaligned above tAe track, as shown in FIG. 6C,
the polarity o~ the error signal may he, for example, positive.
Conversely, if the head is misaligned below the track, as shown
in FIG. 6D, the polarity of the error signal may be assumed to
be negative. This error signal is filtered to a d.c, level by
low pass filter 24 and then is added l:o the dither signal SA to
be supplied as a correcting signal, or tracking control signal,
to piezoceramic support device 19.
Thus, although-head 20 fluctuates, or oscillates, about
its scanning path because of the dither signal SA which is supplied
to piezoceramic support device 19 by oscillator 16, if this scan-
ning path is, for example, displaced above the record track which
is being scanned, as represented by FIG. 6C, phase detector 23
supplies a positive signal to low pass filter 24 w~ich, in turn,
adds a positive d.c. level to the dither signal in adder 17.
The magnitude of this d.c. level represents the displacement of
the actual scanning path of head 20 from its desired scanning
path, i.e., from the track. Consequently, t~e tracking control
voltage which is supplied to piezoceramic support device 19 from
adder 17 appears as a dither signal superimposed onto a pcsitive
d.c. level. Piezoceramic support device 19 deflects downwardly
in response to this posit~ve d.c. level, thereby bringlng the
scanning path of head 20 into alignment with the track. Of course,
the dither signal SA oscillates the head about its scanning path,
as shown in FIG. 6A. Conversely, if head 20 exhi~its a mis~
tracking error in the downward direction relative to the track
being scanned, as xepresented by FIG. 6D, phase detector 23 pro-
duces a negative signal which is supplied, by low~pass filter 24,
as a negative d.c. level to be added to the dither s~gnal in
.
-19 - ' `'
~13~7
adder 17. This negative d.c. level deflects piezoceramic support
device 19 in the upward direction, thereby returning.the scanning
path of head 20 into alignment with t~le track. Of course, here
too, the dither signal S~ oscillates the head about its scanning
path, as represented by FIG. 6A.
The tracking control circuitry thus far described is
effective, genexally, to adjust the position of head 20 relative
to the track being scanned there~y during a normal reproducing
operation. That is, in the aforedescribed operation, change-
over switch 14 is conditioned such that its movable contactenga~es its play~ack contact P so as to couple the ou~put of
adder 17, that is, the d.c. level superimposed onto the dither
signal SA, to piezoceramic support device 19. The tracking -.
control circuit includes an additional section which is operative
during special reproducing modes of operation to compensate for
mis-tracking errors of head 20. ~his additional section thus
corrects tracking errors which may be present during, for example,
slow motion, stop motion or qu~ck motion modes. That is, if the
scanning trace of head 2~ deviates from the record track ~eing
scanned during such special reproducing modes, as represented by
trace 12 shown in FIG. 3, this additional section of the tracking -~
control circuit corrects for such deviation.
The aforementioned additional section of the tracking
control circuit includes a sawtooth waveform generator 26 and a
SPECIAL mode switch 25 (referred to hereinafter me~ely as the
SPECIAL switch). Sawtooth waveform generator 26 generates a
sawtooth shaped waveform of constant period and o~ an amplitude
which is a function of the expected maximum dev~ation o~ the scan-
ning trace of head 20 from the track ~eing scanned.. Since thi~s
deviation is a function of the speed at which the video tape is
-20-
-, - . . . .. .... .
~39L~7
transported, the amplitude ~f the sa~tooth shaped waveform i5,
therefore, a function of the tape speed. For example, and as
shown in FIG. 3, scanning trace 12 deviates from desired scan-
ning trace 13 by a greater amount during stop motion reproduction
than during slow motion reproduction. Hence, the amplitude of
the sawtooth shaped wave~orm generated by sawtooth waveorm
generator 26 should be greater during the stop motion mode than
during the slow motion mo~e. A tape speed detecting circuit is
connected to the input of sawtooth waveform generator 26 for
supplying an amplitude control signal to the sawtooth waveform
generator depending upon the detected tape speed.
In the illustrated embodiment, the tape speed detecting
circuit is comprised of a pick-up head 27 adapted to reproduce
the control pulses CT which are recorded along a longitudinal
edge of tape T, as shown in FIG. 3, a sensor 30 which is adapted
to generate a pulse whenever head 20 rotates into a predetermined
position, and a phase detector 29 which is adapted to detect the
phase differential between the control pulses CT reproduced by
head 27 and the position pulses reproduced by sensor 30. More
particularly, head 27 is connected to one input of phase detector
29 by an amplifier 28, and sensor 30 is connected to another input
of the phase detector. Phase detector 29 supplies an amplitude
control signal to sawtooth waveform generator 26; and this gen-
erator supplies its sawtooth shaped waveform to the output of
adder 17 via SPECIAL switch 25. ;;
During a normal reproducing mode, SPECIAh switch 25 is
opened such that the sawtooth shaped waveform produced by sawtooth
waveform generator 26 is not superimposed onto the output of
adder 17. Furthermore, during this normal reproducing ~ode,
the tape is transported at its normal speed such that a predetermined
-21-
~3~7
phase relation exists between control pulses CT reproduced by
head 27 and the position pulses produced by sensor 30.
If th~ VTR now ii6i disposed in one of its special
reproducing modes, SPECIAL switch 25 is closed. Furth rmore,
khe transport speed of the video tape is changed from its
normal speed. This means that the predeterm;ned phase relation
between the control pulses CT reproduced by head 27 and the
position pulses produced by sensor 30 no longer is present.
This change in the phase relation between the control and posi-
tion pulses is detected by phase detector 29 which, in turn,supplies an amplitude control signal to sawtooth waveform gen-
erator 26 which is a function of the tape transport speed during
this special mode of reproduction. As a consequence thereof,
the amiplitude of the sawtooth shaped waveform produced by saw-
tooth waveform generator 26 is set to a level which corrects themis-tracking error of head 20. That is, the properly set ampli
tude of the sawtooth shapsd waveform is superimposed with the
tracking control voltage produced by adder 17 ~or the purpose of
correcting the scann~ng trace of head 20, and thus bringing it
into alignment with the track then being scanned during this
special mode of reproduction. Change-over switch 14 now supplies
a tracking control voltage to piezoceramic support device 19
which includes the dither signal component S~, the d~c. le~el
derived from phase detector 23 and the properly selected sawtooth
shaped waveform S8 produced by sawtooth waveform generator 26.
These components combine to control head 20 such that the scan-
ning trace thereof is brought into alignment with the track ~eing
scanned during the special mode of reproduction (e.g. the stop
motion, the still motion or the ~uick motion modes), while head
20 continues to fluctuate, or oscillate, about its scanning trace,
as shown in FIG. 6A.
-22-
3~
In addition to the tracking control circuit~y described
hereinabove, FIG. 5A also includes an oscillator 32 which i~
selectively triggered when the VTR apparatus is changed over
~ from its reproducing mode to a stand-by mode, the oscillating
signal produced ~y this oscillator then being supplied ~o piezo-
ceramic support device 19 for the purE)ose of cancelling the
remanence thereof. Oscillator 32 may include a monostable
multivibrator, or one-shot circuit, which is adapted to supply
an operating voltage to the oscillator for a predetermined time
duration, depending upon khe time constant of the monostable
multivibrator. In one embodiment, this monostable multivibrator
may be of the type which generates a pulse signal having a sub-
stantially vertical leading edge and a gradually decaying trailing
edge. When oscillator 32 is supplied with an operating voltage
having this waveform, the oscillator generates an oscillating
signal whose envelope is similarly shaped. That is, during the
predetermined time dura~ion determined by the monosta~le multi-
vibra~or, the oscillating signal exhibits a substantially fixed
constant amplitude during a first portion of this t~me duration,
and then the amplitude of the oscillating signal gradually de-
creases, or decays, during the following portion of the time
duration. Such an oscillating signal having this envelope is
illustrated in FIG. 7, wherein the fixed amplitude thereof is
present during portion PA, and the gradually decreasing amplitude ~;
thereof is present during portion PBo
As another embodiment of oscillator 32, the operating
voltage supplied thereto for the aforementioned predetermined
time duration initially may exhibit a maximum le~el, and then
the amplitude of this operating voltage may gradually decrease
from this maximum level for the entire time duration. That is,
:
in this alternate embodiment, ~he ~aveform of the oscill~ting
signal may appear as shown in FIG. 7 during portion PB. One
of ordinary skill in the art would be fully enabled to produce
an operating voltage having a wave shape similar to khe envelope
of the oscillating signal shown during portion PB.
Oscillator 32, as shown in FIG. 5A, is coupled to a
voltage supply source 31 by a change-over SWitC~l 15 which is
mechanically ganged for simultaneous operation with chang~-over
switch 14. In particular, when the movable contact of change-
over sw tch 14 is in engagement with playback contact P, themovable contact of change-over switch 15 likewise is in engage-
ment with its playback contact P, the latter being isolated from
further circuitry. If change-o.ver switch 14 is operated such
that its movable contact engages its stand-by contact S, thereby
15 coupling the oscillating signal Sc produced by oscillator 32 to
piezoceramic support device 19, the movable contact of change-
over switch 15 likewlse engages i.ts stand-by contact S, thereby
supplying the voltage from source 31 to oscillator 32. It is
appreciated that the abrupt application of voltage rom source
31 to stand-by contact S of change-over switch 15 serves to
trigger the monostable multivibrator included in oscillator 32
such that the oscillating signal generated by this osci~llator
at time tS, that is, at the time that change-over switch 15 is
changed over in this manner, appears as shown in FIG. 7. In
the alternative embodiment of oscillator 3" described above,
when change-over switch 15 is changed over to its stand-by condi-
tion, that is, when its movable contact is brought into engagement i ~;
with its stand-by contact S, the voltage supplied from source 31
results in an operating voltage supplied to oscillator 32 having .
the gradually decreas;ng, or decaying shape of the type exhibited
-24-
3~
by t~e envelope of the oscillating signal shown during portion PB
in FIG. 7.
Thus, in either embodiment o oscillator 32, when
~ change-~ver switches 14 and 15 are changed over from their play-
; 5 ~ack conditions to their stand-~y conditions, ~he ~racking control
voltage which previously had ~een supplied to piezoceramic support
device 19 ~rom adder 17 is replaced by the osci~lating signal
generated by oscillator 32. That is, and as shown more clearly
in FIG. 7, prior to time ts, the VTR apparatus operates in its
reproducing mode, such as ts normal reproducing mode, whereby
dither signal SA is supplied as the tracking control signal to
piezoceramic support device 19. At time tS, the VTR apparatus
is changed over from its reproducing mode to its standby mode,
whereby oscillating signal Sc is supplied from oscillator 32
through change-over switch 14 to piezoceramic support device 1~.
It is fully appreciated that the magnitude of the oscillating
signal Sc, when initially supplied to the piezoceramic suppoxt
device at time tS, is greater than the amplitude Qf the dither
control signal SA which had been supplied to the piezoceramic
support device. Even lf the VTR apparatus had been operating
in its special mode of reproduction, the amplitude of the overall ;
tracking control signal which had been supplied to piezoceramic
support device 19, that is, the amplitude of the sawtooth shaped
~7aveform together with the superimposed d.c. level-adjusted dither
sigr~al, is less than the magnitude o~ the oscillating signal Sc.
When the oscillating signal Sc ic supplied to piezo-
ceramic support device 19, the support device is driven through
its hysteresis loop shown in FI~. 4. That is, its maximum positive
deflection is dependent upon the magnitude of t~e oscillating ;~
signal Sc during the positive half-cycle, and its maximum negative ~;
-25- `
~ll3~ '7
deflection is dependent upon the magnitude of the oscillating
signal during its negative half-cycle. As the magnitude of
the oscillating signal gradually is reduced, as during portion
PB, the positive and negative deflections of piezoceramic
support device 19 likewise are reduced. That is, ~he hysteresis
loop shown in FIG. 4 is made smaller. Finally, when the magnitude
of ~he oscillating signal has been reduced to zero, ~he actual
defiection of piezoceramic support device 19 likewise will have
been reduced to zero. Hence, the remanence of the piezoceramic
support device, which otherwise would prevent this device from-
returning to its home position in the absence of a voltage applied
thereacross, is cancelled. Consequently, at the conclusion of
the predetermined time duration following the change-over of
switches 14 and 15 from their playBack conditions to their ~tand-
by conditions, head 20 will have returned to its home position.
The relationship ~etween the cancellation of the rema-
nence of the piezoceramic support device and the magnitude of
the oscillating signal supplied thereto is graphically depicted
in FIG. &. In this graphical representation, the abscissa repre-
sents th~ magnitude of the oscillating signal and the ordinaterepresents the remanence. For the purpose of the present dis-
cussion, it will be assumed that t~e remanence is the amount of
deflection of the piezoceramic support device in response to a
zero voltage applied thereacross divided by the maximum deflection
2$ which the device may underg~. For the particular type of piezo-
ceramic material which is used as the piezoceramic support device,
it is assumed that the piezoceramic material exhib~ts a remanence
of 20~ when the magnitude of the oscillating signal is zero. As
the initial amplitude of the oscillating signal is raised ~eyond
zero, the remanence is seen to decrease. Thus, if the initial
-~6-
.
,. : .
.. ..
~l~3~7
: amplitude of the oscillating signal i6, for example, llQ volts,
then, at the end of the aforementioned predetermined time dura-
tion, that is, after the magnitude of this oscillating signal
has been reduced to zero, the remanence is reduced to 5%. If
the initial magnitude of the oscillating signal is increased to
220 volts, then, after the predetermined time duration during
which the oscillating signal of gradually decreasing magnitude
is applied, the remanence of the piezoceramic support device
is reduced to about 2~. From ~IG. 8, it is appreciated that,
as the initial amplitude of the oscillating signal is increased
further, a greater amount of remanence is cancelled thereby.
The magnitude of the oscillating signal supplied to
the piezoceramic support device cannot be raised beyond a threshold
voltage Ec, now to be described with reference to the graphical
representation shown in FIG. 9. This graphical representation
illustrates the relationship between the non-responsiveness of
the piezoceramic material to a control voltage applied there- :
across when the control voltage is of a relatively high level~
If the polarity o~ the control voltage is opposite to the polari-
zation of the piezoceramic material, the material will be sub-
jected to expanding forces similar to those which are present
in piezoceramic member 34, discussed previously with respect to `- :
FIG. 1. It is recalled that these forces result in the deflection
of the piezoceramic support device. Genexally, the amount of
such deflection, that is, the responsiveness of the piezoceramic
support device, is related to the magnitude of the voltage applied
thereacross. ~owever, for higher voltage levels, the responsive- ~
ness of the piezoceramic material is reduced. Finally, for a ;.
threshold level Ec, the value of which is established by the piezo- ~-
ceramic material, the piezoceramic support device is fully
~ 3~ 7
non-responsive. FIG. ~ repre$ents the non-responsiveness of
the piezo-cexamic material to the magnitude of the voltage
applied thereacross. In particular, the abscissa of this
graphical representation represents the magnitude of the
applied voltage, and the ordinate represents the percentage
of non-responsiveness. It is seen that, at the threshold voltaye
Ec, the piezoceramic support device is fully (i.e., 100~) non-
responsive. For a voltage whose magnitude is 0-5 Ec or less,
~ the piezoceramic support device is fully responsive (i.e., 0%
10 non-responsive). Betwaen the range O.5Ec and Ec, the non-
responsiveness of the piezoceramic material increases from 0
to 100~.
In selecting the magnitude of the oscillating signal
which initially is supplied to piezoceramic support dev~ce 1
when change-over switch 15 is changed over from its playback
condition to its stand-by condition, the graphical representations
of both FIGS. 8 and 9 must be taken into account. As shown in
FIG. 8, if the magnitude o r this oscillating signal is salected
to be high, the amount of remanence whic~ remains is ~ery low.
However, as represanted by FIG. 9, if the amplitude of the oscil-
lating signal is selected to be too high, the characteristics of
the piezoceramic support device may be substantially disturbed
such that the device no longer is satis~actorily responsive to
a tracking control voltage applied thereacross. That ~s, the ~
non-responsiveness shown in FIG. 9 may be considered to be perma-
nent, even after the oscillating signal is removed~ Hence, if
the magnitude of the oscillating signal is equal to Ec, the piezo-
ceramic support device will be permanently non-responsive to
control voltages, even of very small magnitude, subsequently
applied thereacross.
-2~-
. . .
.- : . , . : .
. ~ , . .
~3~
In view of the constraints presented h~ FIGS. 8 and 9,
; the preferred ranye of the magnitude of the oscillating signal Sc
which initially is supplied to piezoceram-c support device 19 is
between 0.25EC and 0.75EC. If the characteristics of FIGS. 8
and 9 are drawn for the same piezoceramic material, then 0.25EC =
.llOv, and 0.75EC - 330v, wherein ~c = 440v. This range of the
magnitude of the oscillating s;gnal is seen to ~e much greater
than the level of the dither signal SA which is supplied to piezo-
ceramic support device 19 during reproducing modes o~ operation,
which dither signal level is on the order of about O.OSEc.
The relationship between the non-responsiveness of
the piezoceramic support device as a function of the voltage
applied thereacross, as shown in FIG. g, has been prepared as
a result of experimentation wherein each test voltage was applied
across the piezoceramic leaf member for about 500 hours. After
each test voltage was removed, the piezoceramic leaf member was
tested to determine its responsi~eness to a control ~oltage.
The results of this experimentation are represented by the
graph of FIG. 9.
By selecting the magnitude of the oscillating signal Sc
to be within the range 0.25EC to 0.75EC, wherein Ec is the threshold
voltage associated with the particular piezoceramic material which
is used, it is seen that most of the remanence of the piezoceramic
support device is cancelled, and the piezoceramic support device
is made only slightly non-responsive to subsequent control voltages
by reason of the higher magn1tude of the oscillating signal. In
selecting the desired magnitude-of the oscillating signal, it
must be remembered that a greater amount of remanenc~ can be
cancelled only at the cost of increasing the non-responsiveneSs
of the piezoceramic support device to subsequently applied tracking
-29-
~34~
control voltages. Of course, regardless of the particular
magnitud~ of the oscillating signal which is selected, t~s
magnitude should be substantially greater than the level of
the tracking control ~gnal, as represented by FIGS. 7 and ~.
The embodiment of FIG. 5A represents the ma~ner in
which oscillator 32 is used to cancel the remanence o~ piezo-
ceramic support device 19 when the VT~ apparatus is changed over
from its reproducing mode to its stand-by mode. Referring now
to FIG. 5B, the embodiment illustrated therein repr~sents the
manner in which oscillator 32 is used to cancel the remanence
of piezoceramic support device 19 when the ~TR apparatus is
changed over either from its reproducing mode or from its stand-by
mode to its recording mode. The elements illustrated i~ FIG. 5B
which are similar to those described hereinabove with respect to
EIG. 5A are identified by the sa~e reference numerals. ~he only
differences reside in that change-over switch 14l (FIG. 5B) i~-
cludes an additional contact, referred to as its record CQntaCt R,
connected to the output ~f oscillator 32. ~urthermore, stand-by
contact S is electrically ;~solated. This differs from the afore-
described embodiment o~ FIG. 5A wherein change-oYer switch 14
does not include the record contact R and, moreover, has its
stand-by contact S connected to the output of oscillator 3Z.
Furthermore, in FIG. 5B, change-over switch 15' has a record
contact R connected to the input of oscillator 32, and its mov-
able contact is in engagement with the electrically isolated
contact ~S/P) either during the stand-by mode or during the
playback mode. In all other respects, the apparatus illustrated
in FIG. 5B functions in the same manner as discussed previously
with respect to FIG. 5A. Thus, when the VTR apparatus of FIG. 5B
is in its playback mode, the tracking control signal, discussed
-30-
~3~
above, is supplied to pieZoceramic ~upport device 1~. When the
VTR apparatus is c~anged over to its recording mode, oscillator
32 is triggered to generate the oscillating signal Sc, discussed
previously, and having a waveform of the type shown ~y the
5 example of FIG. 7. In the record mode, the output o~ oscillator
32 is coupled via change-over switch 14' to drive ampli~ier 1~,
whereby the oscillating signal Sc is supplied to ~iez~ceramic
support device 19 for removing the remanence th~reof during the
initial portion of the recording mode. At the end of the pre-
determined time duration, that is, when oscillating signal Schas decayed to zero, video signals then can be recorded on the
record medium by head 20 with uniform pitch. The embodiment dis-
cussed with rsspect to FIG. 5B is particularly beneficial when
the VTR apparatus is operated in an edit operation.
It is, of course, appreciated that in the ~mbodimént
of FI~. 5B, the oscillating signal Sc is supplied to piezoceramic
support device 19 when the VTR apparatus is changed oVer from its
playback mode to its recording mode or from it~ stand-by mode to ;
its recording mode. That is, a predetermined time duration a~
the initial portion of the recording mode is dedicated to cancel-
ling the remanence of piezoceramic support device l~.
While the present invention has been particularly shown
and described with reference to preferred embodiments, it should
be readily apparent to those of ordinary skill in the art that
~arious changes and modifications in form and deta~ls may be
made without departing from the spirit and scope of the invention.
For example, the apparatus with which the present invention can ~-
be used need not be limited solely to a video recorder, such as
a VT~. The present invention can be used with other recordi~ng
and/or reproducing devices in which a piezoceramic support device
-31- ;
. .
~L~3gL~Lt~
is used to scan a transducer across a record medium. The record
medium need not be a magnetic tape, nor need this medium ~e
limited to a magnetic recording medium. Furthermore, the track-
ing control circuitry need not be limited solely to the type
shown in FIG. 5A and comprising aither signal source 16, envelope
detector 22, phase detector 23, and sawtooth wa~e~orm yenerator
26.
It is intended that the claims ~e interpreted as includ-
ing the forsgoing as well as other changes and modificat~ons.
--32--
