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BACKGROUND OF THE INVENTION
The present invention relates to an auto tracking apparatus for use in
video tape recorders of the helical scan type having a pair of rotary
heads which are mounted in diametrically opposite positions of a rotary
cylinder.
Various auto tracking apparatus have recently been put to practical use in
the field of video tape recorders in order that a magnetic head can
accurately scan a video signal-recorded track on reproduction,
particularly, speed-varying reproduction such as slow-motion reproduction.
One example of the conventional auto tracking apparatus is disclosed in
U.S. Pat. No. 4,451,860 or Japanese Patent Provisional Publication No.
57-66526, the teachings of which involve sampling and holding the
detection output of a reproduction high frequency signal at a given point
in a field and controlling the head position to keep the level of the
detection output to a maximum. However, such prior art apparatus would
suffer the problem in that difficulty is encountered to ensure effective
tracking control in cases where the following of the track is not parallel
to the center line thereof, because the sampling of the detection output
signal is performed in terms of only one point per each of a plurality of
fields. Thus, a further improvement would be required from the viewpoint
of increasing accuracy of the tracing by a reproducing head.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a new and
improved auto tracking apparatus which overcomes the above-described
disadvantage inherent in the prior art apparatus.
More specifically, the present invention provides an auto tracking
apparatus which makes it possible to accurately trace the whole of a
track.
A feature of the present invention is to divide a tracing time period of
one field of a frame into a plurality of time domains and to sample and
hold the envelope detection output signal of the reproduced high frequency
signal at every time domain. This sample-and-hold operation is
successively made for the following frame and each of the sampled values
is compared with the previously sampled value at the corresponding time
domain. In accordance with the results of the comparison the displacement
of a reproducing head is controlled at every sampling point so that the
reproducing head can accurately scan the track along the center line
thereof.
According to the present invention, there is provided an auto tracking
apparatus comprising: a reproducing head disposed on the circumference of
a rotary cylinder of the video tape recorder, the reproducing head being
mounted on a head drive element and arranged to be movable in the
direction normal to the plane of rotation of the rotary cylinder; a drive
circuit for displacing the reproducing head by driving the head drive
element; a circuit for performing envelope detection of a high frequency
signal reproduced by the reproducing head; first sample-and-hold circuit
means for sampling and holding the output signal of the envelope detection
circuit in each of a plurality of time domains provided by dividing the
duration of one field of a frame; second sample-and-hold circuit means for
sampling and holding the output signal thereof in each of a plurality of
time domains provided by dividing the duration of one field of the
following frame; a plurality of comparison circuits for successively
comparing the outputs of the first sample-and-hold circuit means with
those of the second sample-and-hold circuit means; a plurality of control
circuits for generating control signals for controlling the reproducing
head in accordance with the results of the comparisons; and a circuit for
alternately successively supplying the control signals to the head drive
element.
In accordance with the present invention, there is further provided an auto
tracking apparatus comprising: a reproducing head disposed on the
circumference of a rotary cylinder of the video tape recorder, the
reproducing head being mounted on a head drive element and arranged to be
movable in the direction normal to the plane of rotation of the rotary
cylinder; a drive circuit for displacing the reproducing head by driving
the head drive element; a circuit for performing envelope detection of a
high frequency signal reproduced by the reproducing head; a first group of
three sample-and-hold circuits for sampling and holding the envelope
detection output for a field of a frame at three timings; a second group
of three sample-and-hold circuits for the envelope detection output for
the corresponding field of the following frame at the same three timings;
a first comparison circuit for comparing the sampled value obtained by the
first sample-and-hold circuit of the first group of three sample-and-hold
circuit with the sampled value obtained by the first sample-and-hold
circuit of the second group of three sample-and-hold circuits; a second
comparison circuit for comparing the sampled value obtained by the second
sample-and-hold circuit of the first group of three sample-and-hold
circuit with the sampled value obtained by the second sample-and-hold
circuit of the second group of three sample-and-hold circuits; a third
comparison circuit for comparing the sampled value obtained by the third
sample-and-hold circuit of the first group of three sample-and-hold
circuit with the sampled value obtained by the third sample-and-hold
circuit of the second group of three sample-and-hold circuits; three
control signal generating circuits respectively responsive to the outputs
from the first, second and third comparison circuits, each of the three
control signal generating circuits generating a control signal for
increasing the output level of the drive circuit when the sampled value
obtained by one of the first group of three sample-and-hold circuits is
greater than the sampled value obtained by the corresponding one of the
second group of three sample-and-hold circuits and generating a control
signal for decreasing the output level of the drive circuit when the
sampled value obtained by one of the first group thereof is smaller than
the same obtained by the corresponding one of the second group thereof;
and a control circuit responsive to the control signals from the three
control signal generating circuits, for outputting a combined control
signal for controlling the displacement of the reproducing head in
relation to one field by successively switching the control signals at the
three timings.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects and features of the present invention will become more readily
apparent from the following detailed description of the preferred
embodiments taken in conjunction with the accompanying drawings in which:
FIG. 1 is a block diagram showing a circuit arrangement according to the
present invention;
FIGS. 2a-2c are timing charts useful for describing the embodiment of the
present invention;
FIG. 3 is a schematic illustration of the structure of the upper cylinder
of a rotary cylinder employed for an auto tracking apparatus according to
the present invention;
FIG. 4 is an illustration, partly broken away, of a rotary cylinder
including the upper cylinder of FIG. 3;
FIGS. 5a-5d are useful for describing the relationship between the traces
of a reproduction head and the reproduction signals;
FIG. 6 is an illustration useful for describing the relationship between
the voltage applied to a head drive element and the level of the
reproduction signal;
FIG. 7 is a block diagram showing the circuit arrangement of the
conventional tracking apparatus;
FIGS. 8a-8c are time charts useful for describing the relationship between
the level of the sampled signal and the voltage applied to the head drive
element;
FIG. 9 is an illustration of traces of the reproducing head; and
FIG. 10, consisting of parts a-b illustrates the reproducing signals
corresponding to the traces of FIG. 9.
DETAILED DESCRIPTION OF THE INVENTION
Prior to describing the embodiment of the present invention a brief
description of the prior art apparatus will be first made with reference
to FIGS. 3 through 10 for a better understanding of the present invention.
An upper cylinder 22 shown in FIG. 3 has a pair of reproducing heads 19 and
24 which are disposed in diametrically opposite positions of the
circumference thereof and further has a pair of recording heads 21 and 25.
The reproducing heads 19 and 24 are respectively mounted on first ends of
head drive elements 20 and 23 such as piezoelectric elements, and opposite
ends thereof are respectively fixedly secured on the upper cylinder 22,
whereby the reproducing heads 19 and 24 are displaceable upon application
of a voltage to the piezoelectric elements 20 and 23. The degree of the
displacement depends upon the amplitude of the voltage applied thereto.
The upper cylinder 22, as shown in FIG. 4, is disposed over a lower
cylinder 30 to form a rotary cylinder and is arranged to be rotatable
relative to the lower cylinder 30 by means of a drive motor 31. Such an
arrangement enables the reproducing heads 19 and 24 to trace parallel
tracks formed obliquely to the longitudinal direction of a tape 29 and
allows the reproducing heads 19 and 24 to be movable in the direction
normal to the longitudinal direction of the tracks.
FIGS. 5 and 6 are diagrams illustrating the relationship among a trace of a
reproducing head, a voltage applied to a head drive element, and a
reproduced high frequency signal (which will be hereinafter referred to as
RF signal). In FIG. 5, the reference characters D1 through D5 represent
the traces of a scan by a reproducing head and the character curve
designated by HSW designates a head switching signal. It will be seen that
the trace position relative to each of the tracks is varied in accordance
with the variations of the amplitude of voltage applied to the head drive
element and the level of RF signal is also varied in accordance with the
variations thereof. When the voltage corresponding to the trace D3 is
applied thereto, the track is accurately scanned by the reproducing head
and therefore the RF signal has the highest level. Namely, the relation as
shown in FIG. 6 is satisfied between the level of RF signal and the
applied voltage. With this fact the auto tracking can be performed by
displacing the reproducing head to keep the level of RF signal to a
maximum.
FIG. 7 is an illustration of a circuit arrangement of the conventional auto
tracking apparatus.
The conventional auto tracking apparatus of FIG. 7 is incorporated with a
video tape recorder (VTR) 32 having a rotary cylinder as shown in FIGS. 3
and 4 wherein a reproducing head can be moved in the direction normal to
the longitudinal direction of the track. A RF signal from the VTR 32 is
supplied to a detection circuit 33 to perform the envelope detection. An
envelope detection output of the detection circuit 33 is sampled and held
by sample-and-hold circuits (S and H CKT) 34 and 35 in response to a
sampling pulse generated by a sampling pulse generating circuit 39. For
example, an envelope detection output in the odd field of a frame is
sampled and held by the sample-and-hold circuit 34, and after an applied
voltage to the head drive element is varied, an envelope detection output
in the odd field of the next frame is sampled and held by the
sample-and-hold circuit 35. Both sampled levels are compared by a
comparison circuit 36. In accordance with the results of the comparison, a
control circuit 37 controls voltage to be applied to the head drive
element, i.e., the direction of displacement and the amount of
displacement of the head. The output of the control circuit 37 is added to
a head drive signal produced by a head drive signal generating circuit 40
on the basis of CTL signal, HSW signal and capstan FG signal from the VTR
32 and then supplied to a drive circuit 38 for amplification.
FIG. 8 is a timing chart illustrating the relationship between the level of
the sampled output signal and the applied voltage. It will be understood
from FIG. 8 that when the level is increased as compared with the level of
the previous frame, the applied voltage is controlled to increase, and
when the level is decreased, the applied voltage is decreased. That is,
the applied voltage is controlled so that the level is kept to a maximum.
Although satisfactory for performing the tracking control in cases where
the trace of a reproducing head is parallel to the center line of the
track, it is difficult to effectively perform the auto tracking
throughout, one field when the trace is not parallel as shown in FIG. 9.
In FIG. 12, the character (a) represents the waveform of RF output signal
obtained by trace D8 in FIG. 9. When the auto tracking is performed using
the conventional auto tracking apparatus, the reproducing head traces the
track as indicated by the character D7, and the trace indicated by D6
cannot be obtained. The character (b) of FIG. 10 designates the waveform
of the RF output signal obtained by the trace D7.
Referring to FIG. 1, there is illustrated an auto tracking apparatus
according to an embodiment of the present invention.
The embodiment shown in FIG. 1 is similar to the FIG. 7 prior art apparatus
with the exception that it includes a plurality of pairs of
sample-and-hold circuits, a plurality of comparison circuits, a plurality
of control circuits, and a switching control circuit. In FIG. 1, a circuit
arrangement for one reproducing head is illustrated for the purpose of
simplicity of a description, and is illustrated as being provided with
three pairs of sample-and-hold circuits whereby the sampling operation of
the envelope detection signal is performed in each of three time domains
per one field.
A RF signal from a VTR 1 is supplied to an envelope detection circuit 2 and
a HSW signal therefrom supplied to a sampling pulse generating circuit 9.
The envelope detection output is sampled and held by sample-and-hold
circuits 3, 4, 10, 11, 15 and 16 in response to sampling pulse signals S1
through S6 from the sampling pulse generating circuit 9. More
specifically, the envelope detection output for a field (for example odd
field) of a frame is sampled at three sampling points by the
sample-and-hold circuits 3, 10 and 15, while the envelope detection output
for the field of the next frame is sampled at the same three points by the
sample-and-hold circuits 4, 11 and 16. The sample-and-hold circuits 3, 4
are coupled to comparison circuit 5, the sample-and-hold circuits 10, 11
are coupled to comparison circuit 12, and the sample-and-hold circuits 15,
16 are coupled to comparison circuit 17. Therefore, for example, the
sampled value held by the sample-and-hold circuit 3 is compared with that
held by the sample-and-hold circuit 4 by means of the comparison circuit
5. The comparison circuits 5, 12 and 17 are respectively coupled to
control circuits 6, 13 and 18 each being provided for controlling voltage
to be applied to the head drive element and the amount of displacement of
the head. Control signals V1 to V3 from the control circuits 6, 13 and 18
are supplied to a switching control circuit 7 in which each of the control
signals is successively outputted at a point corresponding to each of the
sampling points. The output signal from the switching control circuit 7 is
added to a head drive signal produced by a head drive signal generating
circuit 14 on the basis of CTL signal HSW signal and capstan FG signal
from the VTR 1, and then supplied through a drive circuit 8 for
amplification to the VTR 1 for controlling the voltage to be applied to
the head drive element.
An auto tracking operation based on the foregoing arrangement shown in FIG.
1 will be better understood with reference to FIG. 2.
In FIG. 2, three sampling pulses S1, S2 and S3 are generated at an interval
for the duration of one field of the first frame and the other three
sampling pulses S4, S5 and S6 are also generated at the same interval for
the duration of one field of the second frame. First, an envelope
detection output for the field of the first frame is successively sampled
at three sampling points in response to the sampling pulses S1, S2, S3,
and then an envelope detection output for the same field of the second
frame is similarly sampled in response to the sampling pulses S4, S5, S6.
The levels of the outputs sampled in response to the pulses S1, S2, S3 are
respectively compared with those of the outputs sampled in response to the
pulses S4, S5, S6 by means of the comparison circuits 5, 12, 17. In
accordance with the results of the comparisons, the control circuits 6,
13, 18 generate control signals V1, V2 and V3 for controlling the voltage
to be applied to a reproducing head drive element. The applied voltage is
independently controlled at every sampling point so that the amplitude
thereof is increased when the level of a sampled output is greater than
that of the previously sampled output and it is decreased when the level
is smaller than the previous level. With this control the level of the
envelope detection output is kept to a maximum. The control signals V1,
V2, V3 from the control circuits 6, 13, 18 are successively outputted from
the switching control circuit 7 at the timings corresponding to respective
sampling points. That is, a control signal V4 is supplied from the
switching control circuit 7 to the VTR 1.
It should be understood that the foregoing relates to only a preferred
embodiment of the invention, and that it is intended to cover all changes
and modifications of the example of the invention herein chosen for the
purpose of the disclosure, which do not constitute departures from the
spirit and scope of the invention.
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