Note: Descriptions are shown in the official language in which they were submitted.
, 1153~10
ROTARY TRANSDUCER HEAD ASSEMBLIES
This invention relates to rotary transducer head assemblies and more
particularly, but not exclusively, to rotary magnetic head assemblies for
helical scan video tape recorders (HVTRs).
Recently, automatic head tracking systems have been used in HVTRs.
Such head tracking systems use a deflectable magnetic head mounting
assembly on which a video magnetic head is mounted, and the assembly can
be controlled so that the magnetic head scans a video tape at different
angles relative to the length of the video tape. Control of the angle is
necessary to keep the magnetic head in accurate alignment with a track to
be reproduced, most especially when the video tape is transported at a speed
other than the normal reproduction speed or even when the video tape is
stopped for the purpose of still mode reproduction. Thus, such an assembly
permits maintenance of the required scan angle during so-called special
reproduction modes, such as still, slow, fast and even reverse reproduction,
but nevertheless there is a problem with such assemblies. This problem
arises because the assembly uses a single bi-morph leaf, made for example
of piezoelectric ceramic material, one end of the leaf being secured to a
rotary member and the magnetic head being secured to the other end of the
leaf. The bi-morph leaf arrangement is very simple and so long as the
deflection angle of the bi-morph leaf is small, good head-to-tape contact is
maintained. However, when a more substanti~al angle of deflection is
required, for example when the video tape is being transported at more than
twice the normal reproduction speed or is being transported in the reverse
direction, the tape-contacting surface of the magnetic head is no longer
aligned with the surface of the video tape and there is consequent
deterioration in the quality of the reproduced signal.
According to the present invention there is provided a deflectable
transducer head mounting assembly comprising:
a first deflectable member;
a second deflectable member mounted on said first deflectable mernber and
which deflects in response to deflection of said first deflectable member;
and
a transducer head for transducing signals and mounted on said second
deflectable member.
1~53110
According to the present invention there is also provided a rotary
magnetic head assembly for a helical scan video tape recorder, the assembly
comprising:
a tape guide arrangement for guiding a video tape, the arrangement
including coaxial rotatable and fixed drums;
a pair of parallel-aligned bi-morph leaves secured to said rotary drum;
a pair of elastic portions fixed on respective free ends of said pair of bi-
morph leaves;
a rigid head base portion bridging said pair of elastic portions; and
a transducer head mounted on said head base portion.
The invention will now be described by way of example with
reference to the accompanying drawings, throughout which like references
designate like elements, and in which:
Figure 1 is a side elevation of a tape guide arrangement;
Figure 2 is a view of a video tape with recording tracks shown
thereon;
Figure 3 is a cross-sectional view of part of a previously proposed
rotary magnetic head assembly;
Figure 4 shows a detail from Figure 3;
Figure 5 is a cross-sectional view of part of an embodiment of rotary
magnetic head assembly according to the invention;
Figure 6 is a detail from Figure 5 indicating the operation;
Figure 7 is a graph for explaining the operàtion of the embodiment of
Figures 5 and 6; and
Figures 8, 9A, 9B and 9C are enlarged views of modified forms of a
part of the embodiment of Figures 5 and 6.
The embodiment to be described will be assumed to form part of an
HVTR of the one head omega-wrap type, although the invention is not
limited to this application. As is well known, a one inch video tape format
called type C is authorized by the SMPTE, and in this format the video tape
is wrapped around a tape guide assembly through an angle of 340 degrees.
HVTRs using the type C format include a sync head and a video head for
recording and/or reproducing vertical synchronizing signals and the video
portions respectively of a television signal. For the purpose of special
reproduction modes, that is to say still, slow, fast or reverse reproduction,
an additional reproducing video magnetic head is provided, this magnetic
~53110
head having a deflectable mounting. When this additional video head is in
operation, only the video portion of the tracks on the video tape are scanned
and reproduced, and the sync track portion is not reproduced.
The invention and hence the present description is particularly
concerned with the deflectable mounting assembly for such an additional
head.
Referring now to Figure 1, a video tape 1 is wrapped around a tape
guide arrangement 2, the angle of wrap being approximately 340 degrees and
being determined by a pair of tape guides 3 and 4. The tape guide
arrangement 2 also includes an upper rotatable drum 5 and a lower fixed
drum 6, the rotatable drum 5 being rotated by a motor 7 at sixty revolutions
per second in the case of an NTSC television signal or at such other speed as
is appropriate to the television signal being handled, which speed may for
example be fifty revolutions per second for a PAL television signal. ln
Figure 1 the rotating direction of the rotatable drum 5 and the normal tape
transport direction are indicated by arrows A and B, respectively. In the
periphery of the rotatable drum 5 there is provided a head window 8 through
which projects a video head 9 to contact the video tape 1.
Figure 2 shows a pattern of video recording tracks 10, 11 and 12 on
the video tape 1, and also a head scanning locus 15. If it is assumed that the
video tape 1 is being transported in the normal direction at the normal
reproducing speed, then the video head 9 may scan the track 11. On the
other hand, when the video tape 1 is stopped, the video head 9 may follow
the head scanning locus 15 which starts on the track 11, crosses the guard
band between the tracks 11 and 10 and finishes on the track 10. Clearly this
results in unsatisfactory reproduction, and to counteract this the video head
9 is constrained to move at a smaller scanning angle relative to the length
of the video tape 1, by control of a deflectable head assembly on which the
video head 9 is mounted, so that the video head 9 merely scans the track 11
as before.
An example of such a previously-proposed deflectable head assembly
16 is shown in Figure 3, wherein the video head 9 is mounted at the free end
of a bi-morph leaf 17. The bi-morph leaf 17 is secured to the rotatable
drum 5 together with a cover member 18 by a screw 19. Disposed between
the cover member 18 and the body of the rotatable drum 5 so as resiliently
to grip the bi-morph leaf 17 is a rubber block 2~ for damping the vibration
1153110
of the bi-morph leaf 17.
Figure 4 shows a detail from the deflectable head assembly 16 of
Figure 3, illustrating a problem which arises in operation. Normally the bi-
morph leaf 17 is not deflected and so the gap of the video head 9 is
presented perpendicularly to the surface of the video tape 1. In other
words, the tape-contacting surface of the video head 9 is aligned with the
surface of the video tape 1. However, when the bi-morph leaf is deflected
to an extreme position as indicated in broken lines in Figure 4, the magnetic
gap of the video head 9 is no longer presented perpendicularly to the surface
of the video tape 1, that is to say the tape-contacting surface of the video
head 9 is no longer aligned with the surface of the video tape 1 but is at an
angle to the surface of the video tape 1, with subsequent detriment to the
quality of the signal reproduced by the video head 9.
An embodiment of deflectable head assembly according to the
invention will now be described firstly referring to Figure 5. In the
embodiment, the video head 9 is mounted on a deflectable head assembly 21
comprising a pair of parallel-aligned bi-morph leaves 22 and 23 spaced apart
by a spacer 24 and secured together with a cover member 25 to the body of
the rotatable drum 5 by a screw 26. Secured to the free end of the bi-morph
leaves 22 and 23 is a self-deflectable head mounting member 27 to which
the video head 9 is attached. The head mounting member 27 includes a rigid
block portion 28 and an elastic portion comprising two elastic plates 29
respectively rigidly secured to the bi-morph leavès 22 and 23. When the bi-
morph leaves 22 and 23 are deflected by the application of a control signal
thereto, the elastic plates 29 are deflected in the opposite direction,
because the respective elastic plates 29 are rigidly secured to the rigid
block portion 28 and to the bi-morph leaves 22 and 23. This is indicated in
broken lines in Figure 6, and the effect of this self-deflecting action of the
head mounting member 27 is to maintain the magnetic gap of the video head
9 in substantially perpendicular relation to the surface of the video tape 1.
In other words, the tape-contacting surface of the video head 9 remains
aligned with the surface of the video tape 1 when the bi-morph leaves 22
and 23 are deflected in response to the control signal supplied by a control
signal generator 30.
Figure 7 shows an experimental bending curve of an example of the
deflectable head assembly 21. In Figures 6 and 7 the dirnensions A, B and C
llS3110
respectively represent the lengthwise dimensions of the bi-morph leaves 22
and 23, of the elastic plates 29, and of the rigid block portion 28. In Figure
7 the abscissae represent distance in the lengthwise direction of the
deflectable head assembly 21 and ordinates represent distances in microns
measured from the normal position of the top surface of the bi-morph leaf
22 and the head mounting base 27. Thus this distance or deflection is zero
where the bi-morph leaf 22 meets the rotary support on which it is mounted,
and the maximum deflection, that is to say the deflection of the video head
9 is approximately 280 microns. It will be noted that the shape of the curve
in Figure 7 corresponds to the deflected shape of the deflectable head
assembly 21 in Figure 6 and indicates that the tape-contacting surface of
the video head 9 remains aligned with the surface of the video tape 1 when
the bi-morph leaves 22 and 23 are deflected.
ln the particular experiment represented in Figure 7 the dimensions
A, B and C were 18, 2 and 1.5 millimetres respectively, and an alternating
current of amplitude 200 volts and frequency 60 Hz was applied to the bi-
morph leaves 22 and 23, so that the deflectable head assembly 21 vibrated
at 60 Hz.
Figure 8 shows a possible construction of the deflectable head
assembly 21 in more detail. In particular, the shapes of the bi-morph leaves
22 and 23 are such that the outer two-thirds or so of their length, that is to
say the portions of their lengths extending beyond the cover member 25 are
of trapezoidal shape in plan, the two opposite edges being inclined inwards
at the same angle. The head mounting member 27 is formed of a resilient
material such as carbon fibre material, the elastic plates 29 being secured
to the respective bi-morph leaves 22 and 23 by adhesive. The head mounting
member 27 may comprise separate elastic plates 29 and rigid block portion
28 secured together by adhesive. Alternatively, a unitary block of material
may be built up by stacking a plurality of laminates of carbon fibre material
and the final shape of the head mounting member 27 achieved by cutting out
material from the block to leave the space between the elastic plate 29.
Figures 9A to 9C show respective further examples of the
construction of the head mounting member 27. In Figures 9A and 9B the
head mounting member 27 comprises two elastic plates 29 to which is
secured a rigid block portion 28 on which the video head 9 is mounted. In
Figure 9C the head mounting member 27 comprises two elastic plates 29
1~5311~
which are shaped to receive the video head 9 directly, that is to say without
a rigid block portion 28. In each of these three examples it will be noted
that the operative portions of the elastic plates 29 extend in the vertical
direction, that is to say normal to the length of the bi-morph leaves 22 and
23.
Various other modifications are of course possible. Thus in the
above-described embodiment the bi-morph leaves 22 and 23 are piezo-
electric ceramic devices which made by stacking two mono-morph leaves.
As an alternative, however, one of the bi-morph leaves 22 or 23 can be
replaced by an elastic plate which will of course follow the deflection
imposed on the other bi-morph leaf 23 or 22 by the signal from the control
signal generator 3û.
