Note: Descriptions are shown in the official language in which they were submitted.
~0789S9
1 This invention relates to rotary head type
magnetic video recording and reproducing systems
(hereinafter referred to as VTR). More particularly, it
concerns a construction for automatically controlling
the tracking in playback to the optimum tracking state.
In the VTR, it is necessary that in playback
the rotary heads accurately trace record tracks of ~
signal recorded during recording. Accordingly, it has -
been in practice to record during recording a signal
related to the vertical sync. signal of the video signal
as control signal (signal related to the rotational
phase of the rotary heads) in the longitudinal direction ~ -
of the tape and to use this control signal in playback
for controlling the positional relation between the
rotary heads and tape, that is, for tracking control
such that the rotary heads trace the same tracks as at
the time of recording. Hitherto, for adjusting the
tracking to an optimum state where the rotary heads
trace the record tracks most accurately, the phase of
the reproduced control signal has been adjusted by
manually adjusting the phase of a tracking shifter
comprising a monostable multivibrator through a variable
resistor to a position where the signal-to-noise ratio
of image reproduction on a television screen is the
highest.
However, this method of adjustment is very
difficult for the ordinary user. Particularly, it is
difficult to find out a point corresponding to the
highest signal-to-noise ratio, and this leads to
insufficient adjustment and results in lowering of the
- . .... . . . ..
10789S9
1 quality of VTR reproduction. This constitutes a great
drawback with regard to the handling of related
apparatus.
This invention has for its object the provision
of a tracking control system, which can automatically
effect tracking control in playback to the optimum phase
point, thus seeking to improve the operability of the
related apparatus and contribute to the improvement of
the quality of reproduction.
The above and other objects, features and
advantages of the invention will become more apparent ~
from the following detailed description of preferred -
embodiments of the invention when the same is taken in
conjunction with the accompanying drawings, in which:
Fig. 1 shows in block diagram an embodiment
of the invention;
Fig. 2 is a circuit diagram showing an example
of a peak hold circuit;
Fig. 3 is a diagram showing a characteristic
of a comparator used in the embodiment;
Fig. 4 is a graph showing a relation between
the envelope detector output voltage and the phase of a
phase adjustment circuit;
Fig. 5 is a waveform chart illustrating the
operation of some parts of the embodiment of Fig. l;
Fig. 6 shows in block diagram a second
embodiment of the invention;
Fig. 7 is a waveform chart illustrating the
operation of some parts in the second embodiment;-
Fig. 8 shows in block diagram a third
1078959
1 embodiment of the invention;
Fig. 9 is a graph showing a relation between
the envelope voltage and phase of a phase adjustment
circuit used in the third embodiment;
Fig. 10 is a diagram showing a characteristic
of a threshold detector circuit;
Fig. 11 is a diagram showing a characteristic
of a hysteresis comparator;
Fig. 12 is a waveform chart illustrating the
operation of some parts in the third embodlment;
Fig. 13 shows in block diagram a fourth
embodiment of the invention; and
Fig. 14 is a waveform chart illustrating the
operation of some parts in the fourth embodiment.
Fig. 1 shows an embodiment of the invention.
In Fig. 1, there is shown a magnetic tape 1, on which
frequency modulated video signal 2 and also control
signal 3 to be used for tracking servo-control in
playback in the usual way are recorded, and these
signals are reproduced by rotary heads 4 and 4' and a
stationary control signal head 5 respectively.
The rotary heads 4 and 4' are mounted on a head disc 6
which is rotated at a constant r.p.m. by a head disc
motor 8 controlled by a head disc motor control
circuit 7. The magnetic tape 1, on the other hand, is
driven in the direction of arrow 15 from a capstan
motor 9, which includes a frequency generator 10
producing a frequency signal proportional to its r.p.m.
and is controlled by a capstan motor control circuit 11
furnished with the signal from the frequency generator,
-- 3 --
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1 via a pulley 12, a belt 13 and a capstan 14.
The control signal head 5 reproduces the control signal
which is then amplified by a control signal amplifying
circuit 16 for coupling to a phase comparator circuit
17. Coupled to the other input of the phase comparator
circuit 17 is a signal from a phase adjustment circuit
20, which adjusts and shifts the phase of a signal
representing the rotational phase of the rotary heads
4 and 4' obtained through detection of a magnet piece
18 mounted on the head disc 6 by a stationary magnetic
head 19. ~he phase comparator circuit 17 produces an
error signal being coupled to the capstan motor
control circuit 11 to let the capstan motor control
circuit 11 effect fine control of the capstan motor 9
being driven substantially in the neighborhood of a
predetermined r.p.m. so as to control the tape drive
such that the rotational phase of the rotary heads 4
and 4' and the phase of reproduction of the control
signal 3 eventually meet a fixed phase relation
provided by the phase adjustment circuit 20. As a
result, the rotary heads 4 and 4' are rendered to scan
the signal tracks 2 of the record at fixed relative
positions determined by the phase shifter circuit 20.
Consequently, as the reproduced video signal obtained
from the rotary heads 4 and 4' is taken out through a
rotary transformer 21 and coupled through a head
amplifier 22 to an envelope detector circuit 23, the
envelope voltage therefrom is related to the phase of
the phase adjustment circuit 20 in a manner as shown
in Fig. 4.
1078959
1 The output of the head amplifier 22 is also
coupled to a peak hold circuit 24. The peak hold
circuit 24 serves to hold a peak value of the envelope
for a comparatively long period, with decrease of its
holding voltage due to discharging being allowed for,
and it may use a well-known circuit as shown in
Fig. 2, with the time constant for charging of the
envelope detector circuit 23 set to be suitably long
by taking sudden changes of the envelope voltage due
to noise and the like into considerations while setting
the time constant for discharging to be adequately
- long compared to the time constant for charging.
The output of the peak hold circuit 24 and the output
of the envelope detector circuit 23 are coupled to a
comparator 25 which serves as a detecting circuit.
The comparator 25 produces a positive voltage Vl when
Ve 2 vp with Ve being the output voltage of the envelope
detector circuit 23 and Vp being the output voltage
of the peak hold circuit 24 while producing a negative
20 voltage V2 when Ve < Vp, as shown in Fig. 3. In other -
words, when the output voltage Ve of the envelope
detector circuit 23, which has previously been low
compared to the output voltage Vp of the peak hold
circuit 24, is increased with changing phase in the
phase adjustment circuit 20 until it coincides with Vp,
the output of the comparator 25 is changed from the
negative voltage V2 to the positive voltage Vl which
is impressed upon a D-type flip-flop 26 (hereinafter
referred to as D~FF) to be described later.
For changing the phase of the phase adjustment
10789S9
1 circuit 20, a ramp wave voltage is coupled from a ramp
or triangular wave voltage generator 27 through an
analog memory 28 to the phase adjustment circuit 20.
The analog memory 28 is biased with a bias voltage BV
5 applied to its power supply terminal 29, and an input
voltage Vin is coupled to its input terminal 30. When
a control signal appearing at its control terminal 31
is "low", the same voltage as the input voltage Vin
is provided at its output terminal 32. When the control
10 terminal 31 comes up with "high" voltage, the value of
input voltage Vin at the instant of appearance of the
"high" voltage is held and provided as another output
voltage for a long period. That is, the output
voltage at the instant of appearance of the "high"
15 voltage at the control terminal 31 is memorized and
held to prevail irrespective of subsequent variations
of the input voltage Vin. Here, the output of the
ramp voltage generator 27 is coupled to the input
terminal 30 of the analog memory 28, and it is gated
20 under the control of the output of the D-FF 26
coupled to the control terminal 31. While the output
of the D-FF 26 is "low", the output voltage of the
analog memory 28 varies in a ramp waveform in
correspondence to the output of the ramp voltage
25 generator 27, that is, the input voltage at the phase
adjustment circuit 20 varies in the ramp waveform,
causing the phase thereof to vary in the same cycle
as the input ramp voltage from instant A to instant B
as shown in Fig. 4. Also, when the output of the
30 D-FF 26 becomes "high" the control terminal 31 is
1078959
1 adapted to come up also with "high" voltage, so that
the output voltage of the ramp or triangular wave
generator 27 at the instant of appearance of the "high"
voltage at the control terminal 31 is held by the
analog memory 28 as its output voltage, thus holding
the phase corresponding to that voltage.
The function of the D-FF 26 will now be
discussed. In the D-FF 26 the input information is
read-in at the positive edge of a clock pulse, and its
truth value table is as follows.
tn ¦ tn + 1 , ¦
D ~ Q~
Low T Low
High T High ~
When its D input is "high", upon appearance
of a "high" clock pulse at its ck input, its output
becomes "high", and this "high" output continues to
prevail so long as the D input is 'Ihigh". When its D
input becomes "low", its output becomes "low" under
the control of clock pulse coupled to its ck input.
In the instant embodiment~ the output Cl of the afore-
mentioned comparator 25 constitutes the clock pulse
input at the ck input terminal, while an output bl
from a second delay circuit 33 is supplied to the D
input terminal. The output al of a first delay
circuit 3~ is to make the second delay circuit 33 provide
a "low" output for a further period t3 after a period
- t2 which corresponds to the pull-in period of the
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1 servo-system from the commencement of the playback
operation and during which the output al of the first
delay circuit 34 is "low" and after which it becomes
"high" until stoppage of the playback operation,
said period t3 being selected to be long enough to
enable the peak hold circuit 24 to hold the peak value
Vpl of the envelope. This can be readily realized by
so arranging as to detect the voltage at the commence-
ment of the play back operation and also using a delay
line comprising, for instance, a Schmitt circuit.
The sequence of events that are involved in ;
the above construction until the phase of the phase
adjustment circuit 20 is locked at point C in Fig. 4,
representing a phase corresponding to a peak of the
envelope, will now be discussed in detail with
reference to the waveform chart of Fig. 5.
During a period tl from the commencement of
playback operation, during which the output b' of the
second delay circuit 33 is "low", the output of the
D-FF 26 is "low", so that the input at the control
terminal 31 of the analog memory 28 is "low".
In this state, the output voltage of the ramp voltage
generator 27 continues to be supplied to the phase
adjustment circuit 20, and even after the servo-system
pull-in period t2 the phase of the circuit 20 is
changed between points A and B in Fig. 4 during period
t3. By so arranging that a peak value of the envelope
is encountered as the phase is varied between the
points A and B, the peak hold circuit 24 eventually
comes up to hold the peak value Vpl of the envelope.
-- 8 --
1078959
1 At a subsequent instant t4 at which the output cl of the
comparator 25 changes from negative voltage V2 to
positive voltage Vl for the first time after the period
tl, the output dl of the D-FF 26 is inverted to "high",
and from this instant on it is held "high" by a latch
mechanism of the flip-flop and continuously appears as
"high" input at the control terminal 31 of the analog
memory 28. In this way, the output voltage of the ramp
voltage generator 27 at the instant of appearance of
the "high~' voltage is held to prevail. As mentioned
earlier, the voltage of the comparator 25 changes from
negative voltage V2 to positive voltage Vl when
Ve ~ Vp. Since Vp is now the peak value Vpl, the
instant at which the input to the control terminal 31
of the analog memory 28 becomes "high" corresponds to
the instant at which Ve and Vpl coincide with each
other. Thus, the phase of the circuit 20 is locked at
point C (Fig. 4) corresponding to the peak value Vp
of the envelope detector output, thus permitting the
rotary heads 4 and 4' to trace the signal tracks 2
with fidelity.
The period tl mentioned above is the sum of
the servo-system pull-in period t2 and the period t3.
If the peak hold circuit 24 is able to begin to hold
the peak value Vpl of the envelope by the end of the
servo-system pull-in period t2, tl can be reduced to
be equal to t2. Otherwise, the D-FF input bl is made
to be continually "low" for a predetermined period t3
after the servo-system pull-in period t2. By so doing,
the output voltage of the ramp voltage generator 27
1078959
1 can be continually supplied for the subsequent period
t3, during which the phase shift is continued to
permit the peak hold circuit 24 to lay hold of the peak
value Vpl of tne envelope. The periods t2 and t3
may be adjusted by the first and second delay circuits
34 and 33 respectively.
During the period tl from the commencement
of playback, the output dl of the D-FF 26, and hence
the input to the control terminal 31 of the analog
memory 28, remains "low", permitting the ramp voltage
output of the ramp voltage generator 27 to be supplied
to the phase adjustment circuit 20. Therefore, during
the servo-system pull-in period t2 when starting the
playback, the output voltage of the ramp voltage
generator 27 acts as an external disturbance.
This means that the pull-in period t2 for the servo-
system is liable to be extended to extend the
predetermined period t4 required for obtaining correct
and accurate tracking.
To avoid this, the ramp voltage generator 27
may be arranged such that it starts its oscillation
after the servo-system pull-in period t2 under the
control of the output of the delay circuit 34.
With this arrangement, during the pull-in period t2
of the servo-system the output of the analog memory 28
remains the same as a predetermined voltage and does
not act as external disturbance since it is coupled to
the phase adjustment circuit 20, whereby it is
possible to ensure that a predetermined or usual pull-
in period is provided as the servo-system pull-in
- 10 --
1078959
1 period t2. In addition, since in this case the oscil-
lation is started after t2, the instant of appearance of
the ramp voltage at the analog memory 28 after t2 is made
continuous or immediately follows, thus conveniently
eliminating the possibility of disturbance of the servo-
system after t2.
While the instant embodiment makes use of the
ramp wave voltage generator 27 and analog memory 28 as
means for periodically varying the phase of the phase
adjustment circuit 20, it is possible to use any other
suitable means than the ramp voltage generator 27
provided it is capable of periodically varying the phase
of the analog memory 28, and the voltage coupled thereto
need not always be such a ramp voltage but may be a
sinusoidal or saw-tooth wave voltage as well.
The phase of the phase adjustment circuit 20 may
be locked at point C until the playback operation is
stopped in case of self recording and playback where
signals recorded are reproduced by the same system.
However, in the case where signals recorded by different
video tape recorders are joined together, the phase of the
phase adjustment circuit 20 must be shifted during play-
back from point C corresponding to the peak of the
envelope of a signal recorded by one video tape recorder
to another point corresponding to the peak of the envelope
of the next signal recorded by another video tape recorder.
When the envelope of the next signal is
large or small as compared to the envelope of the
proceding signal that has been reproduced with the D-FF
- 30 26 locked at the "high" level, a resetting means 35 is
1078959
1 operated. The resetting means 35 is operated for a
predetermined period t5, whereby the output a1 of the
first delay circuit 34 is made "low" for a predetermined
period t5 + t2. At the same time, a "low" signal is
5 provided by that means to a clear terminal CL of the
D-FF 26 for the predetermined period t5, thus rendering
the output dl of the D-FF 26 "low". Further, during
the period-t5 the peak hold circuit 24 is discharged.
Thus, by operating the resetting means 35 at an instant
10 shown at R in Fig. 5, the output bl of the second
delay circuit 33 is changed from "high" to "low" and
is held "low" for the afore-mentioned period t5 + tl.
After the lapse of the period tl, it again becomes
"high". At the instant R at which the "low" input
15 is made to appear at the clear terminal CL of the
D-FF 26, the output dl thereof is inverted to the "low"
le~Tel, and at a subsequent instant at which the output
al of the first delay circuit 34 becomes "high" again
the oscillation of the ramp voltage generator 27 is
20 started, causing the output el of the analog memory
28 to vary in the ramp wave fashion for a subsequent
period t3, during which the phase is varied within
the phase shift range until the peak hold circuit 24
eventually lays hold of the peak value of the next
25 signal. After the period t3 the outputs of the
envelope detector circuit 23 and peak hold circuit 24
come to coincidence with each other at instant t6,
whereupon the output cl of the comparator 25 becomes
"high", causing the output dl of the D-FF 26 to become
30 "high" again and thereafter continue to be "high" until
1078959
l the playback operation is stopped again or the reset-
ting means 35 is operated once again. In this way,
the output voltage el of the ramp wave generator 27
at the instant t6 is continually supplied to the phase
adjustment circuit 20 to hold the voltage corresponding
a phase for the peak of the envelope of the new signal.
The analog memory 28 which has been employed
in the preceding embodiment as means for holding a
phase for the peak of the envelope, may also be of any
other suitable construction as well so long as such
construction can hold a voltage, which is applied to
the phase adjustment circuit 20 and corresponds to a
phase for the peak of the envelope. For example,
it is possible to use a counter of a construction
comprising flip-flops, and which is adapted to stop
its counting operation at the peak of the envelope.
In this case, the content of the counter at that
instant can be memorized by a latch function of the
flip-flop, and the output of the counter may be
converted through a digital-to-analog converter for
coupling the output thereof to the phase adjustment
circuit 20. Such an arrangement will now be described
in detail with reference to Figs. 6 and 7.
While this embodiment again concerns the
capstan servo-system for controlling the capstan by
the output of the phase comparator 17, modification
for application to the so-called head servo-system
for controlling the rotational phase of the rotary
heads 4 and ~' by the output of the phase comparator
17 is possible, and this is applied to the following
1~789S9
1 embodiments as well.
In Fig. 6 ~ the same component parts as those in
Fig. 1 are designated by the same reference numerals, r
and they are not described here in detail for the sake
5 of brevity of description.
A D-FF 36 operates in the same manner as the
afore-mentioned D-FF 26~ with output c2 (Fig. 7) of
comparator 25 mentioned above coupled to its ck input
and output b2 of a flip-flop 40 to be described later
10 coupled to its D input.
The output d2 of the D-FF 36 is coupled to
a logic circuit 37 ~ which also receives an output a2
of a delay circuit 46 ~ the output a2 being "low"
during a pull-in period Tl of the servo-system from
15 the commencement of playback operation and being
thereafter 'ihigh", and the output of which is coupled
to a terminal 43 of a counter 38
The counter 38 is a so-called up-down
counter, with its input terminal 44 serving to
20 determine the up-counting or down-counting mode.
With a "low" input coupled to this terminal operation
in up-counting mode takes place, whereas a "high"
input to this terminal dictates operation in down-
counting mode. A terminal 42 is a clock pulse input
25 terminal, and the counter 38 counts clock pulses
produced from a clock pulse generator 41 either in
up-counting or down-cGunting mode. A terminal 43
serves to determine whether or not the counting
operation is stopped. When a "high" input voltage
30 appears at this terminal, the counting operation is
~ 14 ~
1078959
1 stopped, while with "low" input voltage the counting
operation is carried on. A further terminal 45 serves
to deliver a single pulse when all the outputs of the
counter 38 becomes "high" or '~low", and with this
pulse signal the flip-flop 40 is inverted. The output
b2 f the flip-flop 40 is coupled to the terminal 44
for determining the counting mode and also to the D
input of the D-FF 36.
By arranging such that closure of the power
source circuit resets the flip-flop 40 "low" and also
renders the output of the counter 38 zero, the counter
38 is caused to start counting from that instant in
the up-counting mode from zero, and as soon as it has
counted the maximum number the flip-flop 40 is inverted
from "low" to "high", causing the counter 38 to start
now operation in the down-counting mode.
The individual bit outputs of the counter 38 ~;
are coupled to a digital-to-analog converter (herein-
after referred to as DAC) 39 for conversion into a
corresponding analog output f2, which is coupled to
phase adjustment circuit 20 for phase shifting therein.
The sequence of events that are involved in
the above construction until the phase of the phase
adjustment circuit 20 is eventually locked at the
phase point C in Fig. 4 corresponding to a peak of the
envelope will now be discussed in detail with
reference to the wavefor~ chart of Fig. 7.
During servo-system pull-in period Tl from
the commencement of playback operation, "high" voltage
is coupled from the logic circuit 37 to the terminal 43
1078959
1 of the counter 38. Thus, during this period the counter
38 is inoperative and provides output of a predetermined
value (for instance zero). In this state, the output
f2 of the DAC 39 has a fixed value, that is, the voltage
applied to the phase adjustment circuit 20 is fixed
so that it does not act as external disturbance upon
the rising of the servo-system. After the period T
the output e2 of the logic circuit 37 becomes "low",
causing the counter 38 to start counting operation,
that is, counting clock pulses coupled to the input
terminal 42 in the up-counting mode from 0 to the
maximum number of, for instance, 255.
If it is so arranged that when the output
voltage of the counter 38 coupled to the phase adjust-
ment circuit 20 is zero the phase thereof is at one endA of the variable range shown in Fig. 4, the phase of
the circuit 20 is shifted with the start of the counting
operation from point A toward the other end B of the
variable range. During this course, the peak hold
circuit 24 is charged to the peak value Vpl of the
envelope at phase C, and from this instant on the value
Vpl is held due to the afore-mentioned time constant
of the peak hold circuit 24. Meanwhile, the output Ve
of the envelope detector circuit 23 turns to decrease
after the phase C is passed due to the afore-mentioned
time constant. Subsequently, as soon as the content
of the counter 38 is increased to the maximum value of
255 (at which instant the phase reaches point B~,
a pulse is produced at the terminal 45 to cause
inversion of the flip-flop 40 to "high", causing the
- 16 -
107B9S9
1 counter 38 to turn to count in the down-counting mode.
At the same time, the D input to the D-FF 36 is
rendered "high", opening the gate for receiving the
output of the coincidence circuit 25. From this
instant the phase is caused to change from point B
toward point A, and the output Ve of the envelope
detector circuit 23 which has previously been reducing
turns to increase again. Upon subsequent reaching of
the phase point C the voltage Ve coincides with the
output Vp being held by the peak hold circuit 24
(i.e., the peak value Vpl of the envelope), thus
causing the output c2 of the coincidence circuit 25 to
change from negative voltage V2 to positive voltage Vl.
At this moment, the output d2 of the D-FF 36 is
inverted from "low" to "high", and thereafter it is
held "high". That is, it continues thereafter to be
coupled through the logic circuit 37 to the terminal 43
of the counter 38 to render the counter inoperative
until the playback operation is stopped. The DAC 39
thus continues to supply as its output f2 the voltage
at the afore-mentioned instant to the phase adjustment
circuit 20 to hold the phase thereof locked at point C
corresponding to the peak of the envelope.
The clock pulse generator 41 used in the
instant embodiment for producing clock pulses coupled
to the counter 38 may be dispensed with to simplify
the construction by so arranging as to use vertical
sync. signal, horizontal sync. signal or control
signal derived from the reproduced signal.
In addition, the use of such signal in lieu of the
- 17 -
10789S9
1 output of the clock pulse generator 41 makes it possible
to detect whether reproduced signal is present or not.
Where no detection as to whether reproduced
signal is present or not is made as in the preceding
embodiment, after the lapse of period Tl from the
commencement of playback operation the counter 38
starts counting operation even in case when no signal
is reproduced from the tape. Even in this case, the
phase of the circuit 20 is again shifted from A to B,
eventually rendering the flip-flop 40 "high" to render
the D input to the D-FF 36 "high" so as to open the
gate for receiving the output of the coincidence
circuit 25, and thereafter the phase is shifted in the
opposite direction from B toward A. When reproduced
signal appears after reversal of direction of phase
shift, the coincidence circuit 25 immediately produces
output since the peak hold circuit 24 has been holding
a low voltage in the absence of reproduced signal
during the phase shift from A to B. Therefore, the
phase which does not correspond to the peak at this
instant is likely to be locked. When reproduced signal
appears during the initial phase shift from A to B,
it is again likely that the peak hold circuit 24
cannot hold the peak of the envelope, thus disabling
the locking of phase corresponding to the peak.
Where vertical sync. signal, horizontal
sync. signal or control signal derived from the
reproduced signal is used for clock pulses, in the
absence of reproduced signal no clock pulse is coupled
to the terminal 42 of the counter 38 even if the
- 18 -
1~78959
1 counter is ready to start counting after completion of
pull-in of the servo-system. In this state the phase
shift does not take place at all, and it is begun
only with appearance of reproduced signal. Thus, it is
possible to reliably lock the phase C corresponding to
the peak without malfunctioning.
In case of using vertical or horizontal
sync. signal for the detection of presence or absence
of the reproduced signal, as a tape portion without
any video signal recorded thereon comes to be scanned
by the rotary heads after pull-in of the servo-system
it is likely that the counter 38 does not make counting
- because of absence of reproduced signal. This incon-
venience can be avoided in case of using control
signal.
Like the previous embodiment using analog
memory 28, the instant embodiment includes a resetting
means 47, which is operated when the envelope is
increased or reduced from that of signal having
previously been reproduced. The resetting means ~7
is operated for a predetermined period T2, whereby
the output of the delay circuit ~6 is made "low" for
a predetermined period T2 + Tl. At the same time, a
"low" signal is provided to clear terminal CL of
the D-FF 36 for the period T2, and which renders the
output of the D-FF 36 "low", while resetting the
flip-flop 40 to "low" to preset the counter 38 such
that it provides zero output, and also the peak hold
circuit 2~ is discharged. As a result, after the
period T2 + Tl the phase is shifted from A toward B
- 19 -
1078959
1 so that the peak hold circuit 2~ comes to lay hold of
the peak of the new signal, and after reversal of
phase shift in the direction from B toward A a phase
corresponding to the peak of the new signal is locked
by the output of the coincidence circuit 25.
With the above construction, the phase of
the circuit 20 is made to reciprocate over its full
variable range. Therefore, the time required until ~ -
locking of the phase is somewhat long, and also during
this time the signal-to-noise ratio of the reproduced
picture is inferior. The time required until locking
of the phase may be curtailed by arranging such that
the direction of phase shift is reversed when the
output of the envelope detector circuit is reduced
after the peak point to an extent that the difference
between the peak value and the output exceeds a
predetermined threshold value.
Such an arrangement will now be described
with reference to Figs. 8 to 12. In Fig. 8 the same
component parts as those in Fig. 6 are designated by
the same reference numerals, and they are not
described any further.
As shown in Fig. 9, which shows a relation
between envelope voltage and phase of the phase
adjustment circuit 20, as the phase is varied from A the
envelope voltage may either be first increased as in
case X or be first reduced as in case Y or Z. In the
former case, the envelope voltage turns to decrease
after it reaches the peak point Cl. In this case,
the direction of phase shift is reversed during the
- 20 -
1078959
1 period of this decrease of envelope. Then, the phase
shift is stopped at the point of coincidence of Ve
and Vp as mentioned above, whereby the time required
until the envelope voltage reaches the peak again
can be reduced. In the latter case, the envelope
voltage that initially decreases eventually turns to
increase and then reaches the peak point C2. In this
case, the direction of phase shift is reversed during
the subsequent period of decrease of envelope, and
the phase shift is stopped at the instant of coincidence
f Ve and Vp. To achieve this~ the output of head
amplifier 22 is also coupled to further envelope
detector circuits 48 and ~9 respectively having
different time constants for charging and discharging,
and the difference between the outputs of these two
detector circuits is amplified by a differential
amplifier 50 and then coupled to a comparator 51 for
comparison with a predetermined voltage.
If the phase of the phase adjustment circuit
20 20 is varied in the direction from A toward B in ~`
Fig. 9 by the output p of the afore-mentioned DAC 39,
in case of phase shift range X where the envelope
first increases the output of the differential
amplifier 50 is positive so that the output of the
comparator 51 is "high". In case of phase shift range
Y or Z where the envelope first decreases the output
of the differential amplifier 50 is negative so that
the output h of the comparator 51 is "low". The output
h of the comparator 51 is coupled to ck input of a
D-FF 52 having similar characteristics to D-FF 36.
- 21 -
1078959
1 Meanwhile, the output of delay circuit 46,
which becomes "high" after the lapse of period Tl from
the commencement of playback operation, is coupled to
D input of the D-FF 52, so that the output i of the
D-FF 52 is inverted to "high" only when envelope is
increased with phase shift after the lapse of period
Tl from the commencement of playback operation.
Further, the outputs of envelope detector
circuit 23 and peak hold circuit 24 mentioned earlier
are also coupled to a threshold detector circuit 53
comprising a comparator. The threshold detector
circuit 53 provides a positive voltage V3 when
Ve > Vp - eO where Ve is the output voltage of the
envelope detector circuit 23 and Vp is the output
voltage of the peak hold circuit 24 and provides a
negative voltage V4 when Ve < Vp - eO. In other words,
it is a circuit for determining whether the voltage
difference between vp and ve is smaller than the
threshold voltage eO (Vp - Ve < eO) g
(Vp - Ve > eO), and it produces as its output the
positive voltage V3 in the former case and the negative
voltage V4 in the latter case, that is, it determines
whether or not the threshold voltage is reached by
the difference between Vp and Ve. Regarding the
characteristics of the threshold detector circuit 53,
if there are fluctuations of envelope voltage of
reproduced signal, envelope voltage difference between
the rotary heads 4 and 4' due to difference in
characteristics between them, a sudden change of
envelope voltage due to noise or other causes and other
1~78959
1 variations while a track of the record 2 is scanned by
the rotary heads 4 and 4', the output voltage of the
comparator is likely to be unsteady and be subject to
fluctuations in the neighborhood of the threshold
voltage input to cause inversion of the comparator,
giving rise to adverse effects upon the control system.
To avoid this, it is desirable to provide a hysteresis
characteristic as shown in Fig. i1. In the character-
istic of Fig. 11, when V2 is increased from a value
providing a large difference between Vp and Ve until
the difference becomes less than el, the comparator
output is inverted to positive voltage V3, but for
inversion to negative voltage V4 again Ve has to be
reduced to an extent that the difference between Vp
and Ve exceeds e2. Thus, it is possible to obtain
steady and stable comparator output characteristic
free from unsteady variations with respect to envelope
voltage ripples and noise less than (e2 - el).
When such a hysteresis comparator is used, voltage e2
corresponds to the threshold voltage eO in case of
Fig. 10. Further, since the outputs of the peak hold
circuit 24 and envelope detector circuit 23 are each
coupled to both threshold detector circuit 53 and
coincidence detection circuit 25, the circuit
construction is simplified.
The output of the threshold detector circuit
53 is coupled together with the output from the
terminal 45 of the counter 38 to a mixer circuit 54.
The mixer circuit 54 produces a positive pulse upon
appearance of output from the threshold detector
- 23 -
1078959
1 eireuit 53, that is, when the envelope becomes lower
than the predetermined threshold value with phase
ehange, and also when the counting mode of the counter
is switched upon saturation thereof. This positive
pulse is coupled through a gate circuit 55 to a clock
input terminal of the flip-flop 56 to render "high"
the output of the flip-flop 56 which has been reset
after closure of the power source circuit. The output
of the flip-flop 56 is impressed upon the input
terminal 44 of the counter 38 to determine up- or
down-counting mode of the counter 38. The function of
the gate circuit 55 will be described hereinafter.
The output j of the mixer circuit 54 is also
coupled to ck input terminal of a D-FF 57 having
15 similar characteristics to the D-FF 36. The D-FF 57
also receives the afore-mentioned output i of the
D-FF 52 coupled to its D input terminal, and it produces
an output k, which becomes "high" only when the mixer
circuit 54 produces output after the envelope turns
20 to increase with change of phase of the phase
adjustment circuit 20. The output k is coupled to
the D input terminal of the D-FF 36, which also
receives the output of the coincidence detection
circuit 25 at its kc input terminal-. Thus, the D-FF
25 36 produces an output m, which becomes "high" only
when the coincidence detection circuit 25 produces
output after reversal of the direction of shift of
the phase of the phase adjustment circuit 20, having
previously reached the point corresponding to the peak
30 of envelope and then been changing toward decreasing
- 24 _
1~78 9S9
1 envelope, back to the direction toward the peak point
again. Logic circuit 37 takes OR from the output m
and the output g of the delay circuit 46 and produces
an output n, which is "high" during the servo-system
pull-in period Tl and also during the "high" period
of the output m of the D-FF 36 and is otherwise "low",
and which is coupled to the input terminal 43 of the
counter 38. The counter 38 is thus rendered
inoperative while the output of the logic circuit 37
is "high" and rendered operative for counting operation
while the output is "low".
The individual bit outputs of the counter 38
are coupled to DAC 39 for conversion into a cor- ~-
responding analog output p coupled to the phase
adjustment circuit 20.
The sequence of events that are involved in
the above construction until the phase of the phase
adjustment circuit 20 is eventually locked at the phase
point Cl or C2 in Fig. 9 corresponding to the peak of
the envelope will now be discussed in detail.
During servo-system pull-in period Tl from
the commencement of playback operation, "high" voltage
is coupled from the output n of the logic circuit 37
to the terminal ~3 of the counter 38, so that the
counter 38 is inoperative and provides a predetermined
value (for instance zero). After the lapse of the
period Tl the output n of the logic circuit 37
becomes "low" (see Fig. 12), causing the counter 38
to start counting operation, that is, counting clock
30 pulses coupled to the input terminal ~2 in the
- 25 -
1078959
1 up-counting mode from O to the maximum number of 255.
If the phase at the instant of start of
counting is as in case X in Fig. 9, the envelope
increases from that instant so that the output h of
the comparator 51 is "high". Thus, the output i of
the D-FF 52 becomes "high" immediately after the
period Tl. As the phase is shifted in the direction
from Al toward Bl the peak hold circuit 24 is charged
up to the peak value Vpl of the envelope at phase Cl,
and it subsequently holds Vpl due to the time
constant set for it. Meanwhile, the output Ve of the
envelope detector circuit 23 decreases after reaching
of phase Cl due to the afore-mentioned time constant.
At a subsequent instant when the difference between
15 Vpl and Ve reaches the predetermined threshold value
eO (instant corresponding to phase Dl in Fig. 9),
the output of the threshold detector circuit 53
becomes "high", causing the mixer circuit 54 to
produce a high pulse coupled through the gate circuit
20 55, which passes the output j of the mixer circuit 54
only when the output i of the D-FF 52 is "high", to
the flip-flop 56 (as indicated by a dotted line in
Fig. 8). With the appearance of the output pulse j
from the mixer circuit 54 the flip-flop 56 is inverted
25 to "high" level, causing the counter 38 to begin its
down-counting mode operation. As a result, the phase
of the phase adjustment circuit 20 turns to be shifted
in the direction from Dl toward Al, so that the output
Ve f the envelope detector circuit 23, having
previously been decreasing, turns to increase gradually
- 26 -
1078959
1 to eventually coincide with the output Vp of the peak
hold circuit 24 (which is at this time the peak value
Vpl of the envelope), causing the output of the
coincidence detection circuit 25 to change from
negative voltage V2 to positive voltage Vl.
Since the D input k to the D-FF 36 has been
"high" from the instant of appearance of the output
pulse of the mixer circuit 54, the output m of the
D-FF 36 becomes "high" at the instant when the output
of the coincidence detection circuit 25 changes to
positive voltage Vl, and from this instant on "high"
voltage continues to be supplied through the logic
circuit 37 to the terminal 43 of the counter 38 until
the playback operation is stopped. Thus, the counter
15 38 is rendered inoperative at this time, and the DAC
39 continues to supply as its output the voltage at
that instant to the phase adjustment circuit 20 until
the playback operation is stopped. In this way, the
phase corresponding to the peak of the envelope is
20 locked.
If the phase at the instant of start of
counting is as in case Y in Fig. 9, the envelope
decreases from that instant so that the output of the
comparator 51 remains "low" though the phase starts
25 to be shifted from A2. Thus, the output i of the
D-FF 52 is also "low". Meanwhile, the output Vp of
the peak hold voltage 24 lS held at Vp2 corresponding
to the phase A2 while the output Ve of the envelope
detector circuit 23 is reduced as the phase is shifted
in the direction from A2 toward B2, and at an instant
1078959
1 when the difference between Vpl and Ve reaches the
predetermined threshold value eO (instant corresponding
to phase D21 in Fig. 9) the output of the threshold
detector circuit 53 becomes "high". However, since
the output i of the D-FF 52 is "low", this output
pulse is not passed through the gate circuit 55 to the
flip-flop 56. ~hus, the flip-flop 56 is not inverted
but continues to provide "low" output. Consequently,
the counter 38 continues counting in the up-counting
mode, continually causing phase shift in the direction
toward C2. As the envelope subsequently turns to
increase after passing the instant F2 corresponding
to the minimum value, output difference is produced
between the envelope detector circuits 48 and ~9
(the difference being produced by appropriately
setting the time constants for charging and discharging
of these two circuits as mentioned earlier) to render
the output of the comparator 51 "high", whereupon the
output i of the D-FF 52 becomes "high". With further
phase shift from F2 to D22 the peak hold circuit 24
is charged up to the peak value Vpl of the envelope
at the instant of phase C2, and with subsequent phase
shift from C2 toward D22 the difference between Vpl
and Ve reaches the threshold value eO at the instant
of phase D22. At this instant the gate circuit 55
passes this time the output pulse j since the output i
of the D-FF 52 has been made "high", thus causing
inversion of the flip-flop 56 from "low" to "high" to
cause the counter 38 to turn to count in the down-
mode. As a result, the phase of the phase adjustment
- 28 -
1078959
1 circuit 20 turns to be shifted in the direction from
D22 toward A2, whereby the output Ve of the envelope
detector circuit 23, having previously been
decreasing, turns to increase gradually so as to
eventually lock the phase of the phase adjustment
circuit 20 to C2 in the manner as described above.
If the phase at the instant of start of
counting is as in case Z where one end B3 of the phase
shift range is close to phase C2 corresponding to the
peak value of the envelope, although the individual
elements operate in the same manner as mentioned so
long as the envelope decreases first, then reaches
the minimum at phase F2 and increases up to the peak
at phase C2, even upon reaching of the phase B3 after
passing phase C2 the difference between the output Vp
of the peak hold circuit 24 (which is the peak value
Vpl of the envelope at this time) and the output of
the envelope detector circuit 23 is not increased
beyond eO. Consequently, the output of the threshold
20 detector circuit 53 is not inverted to the positive
potential, and the flip-flop 56 is not inverted. -
~hus, the counter 38 continues counting in the up-
counting mode. Therefore, the counter 38 will be reset
to zero after it counts the maximum count number, for
25 instance 225, and in such case it is impossible to lock
the phase C2 corresponding to the peak. Accordingly,
the output from the terminal 45 of the counter is
coupled together with the output of the threshold
detector circuit 53 to the mixer circuit 5~ so as to
30 cause inversion of the flip-flop 56 from "low" to
- 29 -
1078959
1 "high" for switching the operation of the counter 38
to the down-counting mode before the counter 38 is
reset to zero by detection of the maximum count number
of 225. In this way, the direction of phase shift is
reversed so that it is shifted in the direction
from B3, which is one end of the phase shift range,
toward C2, thus permitting the locking of the phase
C2 corresponding to the peak of the envelope.
With the construction of Fig. 8, the phase
of the circuit 20 is not reciprocated over its full
variable range but the direction of phase shift is
reversed somewhile after reaching the phase
corresponding to the envelope peak by the output of
the threshold detector circuit, so that it is possible
to reduce time required until the phase corresponding
to the peak value is locked.
In the previous embodiments, the phase of
the circuit 20 is shifted every time when starting the
playback operation of the video tape recorder,
presenting the problem that the time required for
locking the phase is longer than the pull-in period
of the servo-system. Where the signal to be reproduced
is recorded by the same video tape recorder, the phase
corresponding to the peak value of the envelope may be
detected only once and the phase detected may be
directly used when starting subsequent playback
operation.
Fig. 13 shows a further embodiment, which
is provided with a mode selecting circuit 58 for
determining whether playback operation of video tape
- 30 -
10789S9
l reeorder is started for the first time after closure
of power souree eircuit or is started after a previous
playback operation has been stopped, whereby the phase
of the afore-mentioned phase adjustment cireuit 20 is
shifted only in the former case and is not shifted in
the latter case. This is achieved by controlling the
voltage supplied to the delay circuit 46. In Fig. 13,
same component parts as those in Fig. 6 are designated
by the same reference numerals, and they are not
described any further.
The operation of the mode selecting circuit
58 will first be described. A flip-flop (hereinafter
referred to as FF) 59 is reset to "low" by closure of
power source circuit (for instance, by a supply
voltage +P applied upon closure of the power source
circuit), and it is reset to "high" upon stopping of
playback operation of the video tape recorder (for
instance, upon vanishment of a voltage that appears
at the time of start of the playback operation).
It is also reset when the afore-mentioned resetting
means 47 is operated. A gate circuit 60 controls
whether or not a voltage (+V) that prevails during
playback operation is supplied to delay circuit 46 in
accordance with the output q (Fig. 14) of the FF 59.
It passes the voltage prevailing during playback
operation to the delay circuit 46 when the output q
of the FF 59 is "low", but it blocks this voltage
when the output q is "high". Thus, this voltage is
supplied to the delay circuit 46 only at the time of
starting playback operation for the first time after
- 31 -
1078~S9
1 closure of the power source circuit, and once the video
tape recorder is stopped the FF 59 memorises the
stopping so that the voltage is no longer supplied
when subsequently starting playback operation again.
Consequently, the output of the afore-mentioned delay
circuit becomes "high" a delay period Tl after the
commencement of playback operation and becomes "low"
upon stopping of the video tape recorder, as shown at
r in Fig. 14.
When the playback operation is started for
the first time after closure of the power source
circuit, the sequence of events that takes place sub-
- sequently is similar to that described earlier in
connection with the previous embodiment of Fig. 6. -
That is~ after the period Tl the output v of logic
circuit 37 becomes "low", causing counter 38 to start
counting to shift phase of phase adjustment circuit 20
according to the output w of DAC 39, and with the
output t of coincidence detection circuit 25 the output
of the D-FF 36 is locked "high" to lock the phase
corresponding to the peak of the envelope.
When the playback operation is subsequently
stopped, the FF 59 is inverted to "high" as mentioned
earlier, and it holds this "high" state until the
power source is reclosed after it is once opened or
until the resetting means 47 is operated. Thus, even
when playback operation is started again after previous
playback operation has once been stopped, the voltage
(+V) that prevails during playback is not supplied by
the gate circuit 60 to the delay circuit 46, so that
- 32 -
10'~8959
1 "low'` input is made to prevail at the logic circuit
37. Consequently, '!high" input prevails at the
terminal 43 of the counter 38, and the counter is thus
held inoperative and holds its previous count number
that has been held in the previous playback operation.
Since the previous count number corresponds to the
phase C corresponding to the peak of the envelope,
when playback operation is resumed the phase C can be
immediately locked without need of causing phase shift
between A and B. Thus, when resuming playback
operation, locking of the phase C corresponding to the
peak of the envelope can be obtained within the same
period as the ordinary servo-system pull-in period.
In the preceding embodiment, it becomes
impossible for the counter to hold the previous count
number when the power source is disconnected.
This inconvenience can be avoided with the construction
of Fig. 1 using the analog memory 28.
