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SUMMARY OF THE INVENTION
The Adler application, with respect to which the present invention is a
further development, is a novel radial tracking arrangement for optical
image reproducing systems. It has general application to the various types
of video discs presently known including those in which the information is
stored by the technique of pits and intervening lands, by mechanically cut
V-shaped grooves or by photographic techniques with incremental track
areas of variable transmissivity. Of particular interest is the disc in
which the record track includes pits and lands of a uniform width but with
variable longitudinal dimension to represent the temporal variations of a
frequency-modulated signal employed in producing a master of which the
record to be read is a mechanical replication. Desired information may
comprise a video program having luminance, chroma, audio and synchronizing
signals stored in a spiral track each convolute of which represents the
two image fields and timing data characteristic of double interlaced video
transmission employed in commercial television broadcasts.
The pit element of the storage track may have a depth representing
one-quarter wavelength of the reading beam in the medium of the disc or,
alternatively, a depth of one-half wavelength. These differences have
significant bearing on the type image reproducer that is employed,
especially as to the servo arrangement necessary to achieve such controls
as radial beam tracking. Radial tracking for discs with half wavelength
pits required, prior to the Adler improvement, complicated arrangements of
the two tracking beams focused on opposite sides of the storage track and
provided with independent photoreceptors from which a radial tracking
correction signal could be developed. The Adler teaching simplified
matters by obviating the need of extra radial tracking beams. While the
present invention is useful with a variety of video discs it, too, is
especially suited to the half wave pit storage track and, for convenience,
will be described in that connection.
It is an object of the invention to provide a novel focus tracking
arrangement for an optical image reproducing system.
It is another and particular object of the invention to attain focus
tracking for an optical image reproducer wherein radial beam tracking is
accomplished through the technique of wobbling the reading beam relative
to the storage track.
Accordingly, the invention is directed to an optical system having an
information signal stored in a track on a carrier to be derived by reading
of the storage track with a focused beam of energy. A periodic lateral
relative displacement of the beam with respect to the track is effected at
a predetermined wobble frequency to develop a radial tracking error in
response to which radial tracking registration is maintained. The subject
invention improves a system of this type by contributing a focus tracking
arrangement having a pair of photoreceptors, which may also serve for
radial tracking, disposed on opposite sides of a reference plane that
includes the optical axis of the reading beam during radial tracking
registration of the tangent of the storage track. There are means for
obtaining from the output signals of those photoreceptors during operating
intervals in which the beam and carrier are out of focus registration, a
focus signal of the wobble frequency having a phase related to the sense
of focus misregistration and a magnitude related to the extent of
misregistration. The arrangement further includes a synchronous detector
and means for supplying to it the focus error signal as well as a
reference signal phase synchronized to the wobbulation of the reading beam
to develop a focus tracking signal. Additionally, there are means
responsive to the focus tracking signal for maintaining focus
registration.
It is known that video discs of the type under consideration may be
transmissive or reflective and again, for convenience, the transmissive
mode will be described in detail even though the invention has equal
application to operation in the reflective mode.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a schematic representation of a transmissive mode optical image
reproducing system embodying the radial beam tracking arrangement of the
Adler application as well as focus tracking in accordance with the subject
invention:
FIGS. 2 and 3a-3c are sketches employed in describing the operation of
focus tracking;
FIG. 4 is a representation of one form of device that may be utilized in
controlling focus registration; and
FIG. 5 is another embodiment of the invention featuring searching or
sweeping to institute focus tracking.
Referring now more particularly to FIG. 1, the optical image reproducing
system there represented schematically features the use of a laser beam to
read information stored on a transparent video disc 10 of plastic
material, such as polyvinyl chloride, having a storage track in the form
of a multiturn spiral recording information to be derived by reading the
track with a reading beam of energy. The disc is shown in partial radial
cross section for rotation on a spindle 11 driven by a motor 12. Where the
disc is formed of sufficient thickness to be rigid, like an audio record,
it may be supported on a turntable (not shown) for rotation at an
appropriate speed. For some uses there is a distinct preference that the
disc be sufficiently thin, of the order of 6 mils, to be flexible in which
case it is supported and held at a centrally located aperture for rotation
at a high speed in a technique known as flying the disc. Both methods of
support are known and both are useful in systems embodying the present
invention. For the sake of specifity, it will be assumed that disc 10 is
not only transmissive but has been prepared as a replica of a master
recording of an angular-velocity (frequency) modulated carrier signal
modulated in accordance with program information comprising luminance,
chroma and audio components as well as timing or synchronizing components.
Each convolute of the record track represents two interlaced fields of an
image and their associated timing information. The temporal variations of
the recorded carrier signal take the form of a succession of pits and
lands of a common uniform width but variable in longitudinal dimension,
alternating along the record track. The pit dimension will be assumed to
be one-half wavelength of the reading beam in the medium of the record.
Disc 10 is read by a beam produced by a light source 15, such as a helium
neon laser, which forms a collimated beam 3 directed along an optical path
including a beam wobbling device 17. As explained in the Adler
application, it is desirable that the wobble frequency be high in
comparison with the frequency range necessary for effective servo
operation; illustratively a wobble frequency of 100-200 KHz is suitable.
One attraction of the Adler radial tracking arrangement is its simplicity
and inexpensiveness particularly since an acousto-optic element may be
used to accomplish beam wobbulation. The Adler application describes both
a longitudinal mode and a flexural mode acousto-optic element formed of a
glass block to which may be attached one or more piezoelectric elements,
poled and excited to drive the glass block at its natural resonant
frequency. In response to such excitation, a beam traversing the block
encounters an index of refraction which changes in each half cycle of the
resonance to deflect the beam in one direction and then in the opposite
direction a sufficient amount to accomplish the desired lateral
displacement of the reading beam. Usually, it is sufficient to displace
the beam in an amount corresponding to its cross sectional dimension.
Excitation of acousto-optic element 17 is under the control of a reference
oscillator 30 which is tuned approximately to the natural resonance to be
established in the vibrator.
The beam emerging from element 17 is reflected by a mirror 18 that is
displaceable, as indicated by an arrow 19, under the control of a radial
tracking driver 20. Preferably, mirror 18 and its driver 20 are
constructed in accordance with the Adler et al. application, Ser. No.
439,684, filed Feb. 4, 1974, in which case it is a cantilever mounted
piezoelectric bender or bimorph carrying mirror 18 at its free end for
displacement in accordance with the polarity and intensity of an applied
radial tracking signal.
Mirror 18 directs the reading beam to an objective or focus lens 21 which
focuses beam 3 on the storage track of disc 10. The present invention has
to do with focus tracking which may be accomplished by adjusting the
position of the focal plane or the video disc; as shown, it is
contemplated that objective 21 is mounted for movement along the optical
axis of the optical system, being displaceable as indicated by arrow 22
under the control of a focus tracking driver 23. That driver constitutes
means for controlling the position or movement of lens 21 along the
optical path in response to a focus tracking signal to be considered more
particularly hereafter.
Light emerging from disc 10, modulated in accordance with the stored
information, is received by a pair of photoreceptors 35, 36 disposed on
opposite sides of a reference plane 1 that coincides with the optical axis
of the reading beam during conditions of radial tracking registration.
Also at such times the reference plane includes the longitudinal axis of
the segment of the record track instantaneously being read. The outputs of
these photoreceptors are variously used to obtain error signals for
controlling radial beam tracking in accordance with the Adler concept on
the one hand and for controlling focus tracking in practicing the present
invention.
Radial tracking relies on the property that a video disc having half wave
pits effects symmetrical scattering or diffraction of the light of the
reading beam whenever that beam at least partially enters such a pit. The
extent of scattering is dependent on the amount of the beam entering the
pit so that maximum scattering occurs in instants of radial beam tracking
registration. It may be shown, as explained in the Adler application, that
conditions of radial misregistration of the beam relative to the track
manifest phase modulation of that output component of each photoreceptor
occurring at the wobble frequency. The phase of the modulation is related
to the sense of radial misregistration and the magnitude of modulation
reflects the degree of misregistration. Synchronous detection of the
wobble-frequency component in a synchronous detector that is phase
synchronized with the displacement of the beam develops a radial beam
tracking signal appropriate for application to driver 20 to establish and
maintain radial tracking registration. To that end, and since two
photoreceptors are required for focus tracking, the outputs of cells 35
and 36 are applied to a summing amplifier 37 and its output is delivered
to a first synchronous detector 38 to which an appropriate phase reference
signal from source 30 is also applied. The detected output after
amplification and phase compensation in an amplifier 39 is delivered to an
output terminal 40 which may be connected to input terminal 20a of radial
tracking driver 20 to energize that driver and occasion controlled
displacement of mirror 18 to maintain radial tracking registration. Of
course, the output of amplifier 37 also includes a carrier component which
is frequency modulated and is a replica of the frequency-modulated signal
with which the record has been made. That signal may be derived at output
terminal 41 for application to a utilizing device. If that device is a
home television receiver, as is usually the case, terminal 41 may lead to
a transcoder (not shown) which converts the signal derived from reading
video disc 10 into a modulated carrier signal appropriate for application
to the antenna terminals of the color receiver. As thus far described, the
arrangement of FIG. 1 is essentially the same as that described and
claimed in the Adler application, differing essentially only in the fact
that two photoreceptors 35, 36 are shown, whereas only one is required for
radial tracking purposes.
Focus tracking is obtained, in accordance with the subject invention, by
combining the outputs of photoreceptors 35, 36 in a differential amplifier
50 to develop, during operating intervals in which the beam and disc 10
are out of focus registration, a focus error signal of the wobble
frequency having a phase related to the sense of focus misregistration and
a magnitude related to the extent of misregistration. Again, operating
upon that signal in a second synchronous detector 51 develops a focus
tracking signal delivered through an amplifier 52 to the focus tracking
driver 23. Detector 51 also requires a phase reference which is supplied
by the same source 30. Of course, phase compensation and filtering of the
focus tracking signal may be included within unit 52.
The operation of the focus tracking arrangement will be understood quite
readily from a discussion of the sketch of FIG. 2 showing a reading beam
focused in a plane 2 and exciting photocells 35, 36 symmetrically disposed
relative to reference plane 1. The usual convention has been adopted to
represent a cone-shaped reading beam 3. If an object 4 is disposed within
the beam to the left of reference plane 1 it casts a shadow, as indicated
by the construction lines, in the field of photoreceptor 36 and, if the
sum and minus designations shown in the sketch are adopted, the condition
under consideration is one wherein the output of the (+) cell 35 exceeds
that of (-) cell 36. On the other hand if object 4 were to be located to
the other side of reference plane 1, as shown by the broken-line arrow,
the converse conditions are established with the output of (+) cell 35
less than that of (-) cell 36. It will be observed that these conditions
exist where the object is above focal plane 2.
Now consider the case where an object 4' is located within the beam but
below focal plane 2 and to the left of reference plane 1. It decreases the
output of cell 35 relative to cell 36. If it is shifted to the other side
of reference plane 1, again as indicated by the broken-line arrow, the
output of cell 36 is decreased relative to cell 35. In short, out of focus
conditions for objects above and below focal plane 2 produce opposing
changes in the outputs of the two photocells, that is to say, the phase of
the change in signal output indicates the sense in which the object is out
of focus.
This may be viewed a little differently in the manner represented by the
curves of FIGS. 3a and 3b. The curve of FIG. 3a shows the signal obtained
by differentially combining the outputs of the photocells if it be assumed
that an object 4 is moved in a plane above focal plane 2 from a position
just to the left of the beam, through the beam, and then just beyond it to
the right. While the object is within the left half of the reading beam
the output of cell 35 predominates and the differential output is positive
but the output of cell 36 dominates when the object is within the beam and
into the right of reference plane 1, contributing the negative half of the
curve. The curve of FIG. 3b shows the signal derived where the object 4'
is introduced into and disposed through the reading beam from left to
right in a plane beneath focal plane 2. Comparison of the curves of these
two figures shows them to be of opposite phase. Accordingly, detection in
a synchronous detector under the control of a proper phase synchronized
reference signal develops a focus tracking signal of appropriate polarity
and amplitude for application to a focus tracking driver.
An analogous operation takes place in the focus tracking arrangement
included in FIG. 1. More particularly, the described movement of object 4
or 4' in developing the signals represented by the curves of FIGS. 3a and
3b is essentially the same as the movement of reading beam 3 under the
control of acousto-optic element 17 except that the reading beam
displacement is cyclical, occurring at the wobble frequency. Under
conditions of focus misregistration, even if radial tracking registration
be assumed, there is a scattering or diffraction of the light beam as it
scans the record track which develops in detector 51 focus tracking
signals in accordance with the curves of FIG. 3a or 3b, depending upon the
sense or direction of misregistration. Driver 23 responds to that signal
to establish and maintain focus tracking registration.
The amplitude of the focus tracking signal varies with the extent of
defocus in the manner indicated by the curve of FIG. 3c wherein positive
values of the abscissa designate distance above focal plane 2 while
negative values show distance below the focal plane. The unit of the scale
is microns because in the usual video disc the storage track width is
approximately one micron and the diameter of the focused reading beam is
nearly the same. Due to the shape of the focused beam, there is a region
or depth of focus indicated by the zero value of the curve of FIG. 3
immediately adjacent the origin. Since the focus tracking signal has a
polarity related to the sense of focus misregistration, that signal may be
employed to displace an element both sides of a reference to obtain focus
registration.
It is known, for example, that the portion of the disc exposed to the
reading beam may be displaced relative to the focal plane to establish
focus or, alternatively, an optical element may be controlled to adjust
the location of the focal plane. For example, a flexible mirror having an
adjustable curvature may be controlled for that purpose or a cantilever
supported bimorph, having one surface prepared as a reflector, may be
similarly used. Another arrangement, shown in FIG. 4, which suggests
itself is a motor structure quite similar to that of a loudspeaker having
a permanent magnet 55 supported within a magnetic structure 56. In the gap
57 there is the usual coil 58 having an input terminal 59 to which the
focus tracking signal may be applied. The coil form 60 serves as a support
for a lens system 61 and additionally resilient supports 62, 62 may be
provided, holding the lens system for movement along the path or axis of
reading beam 3. The position of the lens is adjustable in accordance with
the polarity and magnitude of the signal applied to coil 58 in known
fashion. No claim of invention is predicated on the structure of the
movable element which is controlled to adjust focus registration. A number
of such devices are well known in the art.
The modification of the invention represented in FIG. 5 adds a focus search
or sweep feature in which a source 70 may apply an alternating current
signal through a switch 71 to focus tracking driver 23. With source 70
controlling the driver, the focus conditions vary over a wide range and
switch 71 is arranged to disable the search feature and transfer focus
adjustment to detector 51 when the condition of focus shall have been
brought within the narrower pull-in range of the focus tracking
arrangement. The switch 71 may normally couple source 70 to amplifier 52
and may respond to an applied control signal that exceeds a threshold
level to switch source 70 out of functional connection with the focusing
system and substitute detector 51 in its place. A suitable control may be
exercised by an amplitude modulation detector 73 to the input terminal 74
of which the output of summing amplifier 37 is applied. When the focus
conditions are close enough to establish at least a minimum RF output from
photoreceptors 35, 36, the output of AM detector 73 will exceed the
controlling or threshold level of switch 71 to enable detector 51 and its
control of focus tracking.
While a particular embodiment of the present invention has been shown and
described, it will be obvious to those skilled in the art that various
changes and modifications may be made without departing from the invention
in its broader aspects. Accordingly, the aim in the appended claims is to
cover all such changes and modifications that may fall within the true
spirit and scope of the invention.
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