Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF TEI:E INVENTION
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Field of the Invention
This invention relates generally to a servo system
for controlling the rotational phase of a rotating body, and is
directed more particularly to a servo system which employs a
digital servo loop to control the rotational phase of the rotary
head drum in a VTR or video tape recorder.
Description of the Prior Art
The rotational phase of a rotary head drum used in
a VTR can be servo controlled either by an analog servo circuit
or by a digital servo circuit.
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A typical analog phase control servo provides an
analog signal, for instance, a DC voltage level, which
corresponds to ~he difference in phase between a reference
pulse and a pulse corresponding to the actual phase or
rotational position of the rotary recording or reproducing
heads. The analog signal is, in turn, used to control the
speed or phase of the rotary heads.
A phase compensation circuit is included in the
analog servo circuit, to emphasi2e the lower frequency phase
deviations in order to compensate for the relatively slow
phase changes caused by DC drift. This circuit normally
incorporates an integration circuit, having a relatively long
time constant, which is used for forming trapezoidal waves
with relatively long slope portions that are sampled by a
reference pulse. ~Jhile the foregoing circuit compensates for
relatively slow changes caused bq DC drift, such a circuit
has a poor transient response. The analog servo circuit is
thus insensitive to the relatively fast phase fluctuations that
produce jitter and picture insta~ility. However, the analog
servo circuits are generally quite simple in construction and
inexpensive as compared with typical existing digital phase
control servos.
A typical digital phase control servo circuit includes
a means to digitize the difference in phase between a reference
p~lse and a control pulse generated in synchronization with the
rotation of the rotary head drum of the VTR. The digitized
signal so produced is used to control the speed or phase of the
rotary head drum. Such a digital servo is able to respond to
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slow changes in phase without the necessity of the long-
time-constant phase com~ensator. The transient response
is improved substantially as compared with that of an analog
servo circuit.
Unfortunately, in a digital phase serva, quantizing
errors will occur, because the phase error is quantized to the
nearest binary number, and the digital phase servo provides
phase correction signals as a voltage stepped in discrete
levels. Quantizing errors result when the voltage necessary
for phase correction lies between two discrete voltage steps.
Such quantizing error results in so-called quantizing noise
which, in turn, produced jitter and degrades the reproduced
picture quality in the VTR.
In order to suppress jitter caused by quantizing
n~ise, it has been necessary to decrease the size of the stepped
voltage levels by increasing the number of bits in the quantized
phase correction signal. At the onset of rotation, or at any
time that the recording head has scanned an unrecorded portion
of tape and beings to scan a recorded portion of tape, the
rotational phase of the head is likely to be severly out of
phase with the reference signal. In order to bring the rotary
heads into correct rotational phase while ~aintaining the
desired reduced vol~age level steps, it has been necessary to
further increase the number of bits for respondinG to such
large deviation.
For example, if high-speed phase compensation
quality similar to that of an analog servo circuit is to be
obtained, the bit number of the digitizing means must be on
the order of fifteen to twenty bits, which adds considerably
to the construction, and cost of the circuit.
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O~Jr~CTS AND SU~RY OF ~IE I~ TIOM
Accordingly, it is an object of this invention to
provide a simple and inexpensive servo system for controlling
the rotational phase of a rotating body, such as the rotar~
recording and re~roducing heads of a video tape recorder,
and which is responsive to both low frequency phase errors,
such as DC drift, and high frequency phase errors, such as
jitter.
A more specific object of this invention is to
provide an improved digital phase servo for controlling the
phase of rotary recording and reproducing heads in a video
tape recorder and which is responsive to low frequency phase
fluctuations including those due to DC drift~ and also
responsive to high frequency phase fluctuations, such as those
associated with jitter, while ~inimizing the effects of
quantizing noise normally associated with digital servos, in
order to provide improved picture stability in the video tape
recorder.
In accordance with an aspect of this invention, a
servo system is provided with a reference signal which is
compared in phase with a command signal corresponding to the
rotational phase of the rotating body to produce a binary level
of "O" or "1" depending upon whether the rotational phase of
the rotating body leads or lags the phase of the reference
signal. The value of the binary level determines whether
the count of a digital counter is increasel or decreased and
the phase of the rotating body is advanced or retarded at
each revolution in dependence on the count of the digital
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counter. After the rotating body is brought into phase with
the reference signal, a stable phase-locked condition is
maintained. Once the correct phase is achieved, a requency
divider operates to prolong the interval between successive
phase corrections, thereby minimizing the quantizing noise
associated with the digital servo.
In one embodiment of the invention, the digital count
of the counter controls a variable delay acting on pulsPs
from a sweep pulse generator which indicate the rotational
phase of the rotary recording and reproducing heads of a
video tape recorder, and the pulses so delayed are applied to
a drive control circuit which controls the speed of a motor
used to drive the rotary heads, so that varlation of the pulse
delay effects the necessary phase correction to the rotary
heads.
In another embodiment of the invention, an analog
loop circuit is included to control the rotational phase of
the rotary heads, with the result that relatively high
frequency phase errors, such as jitter can be accurately
controlled, and phase positioning can be carried out more
precisely. The analog loop circuit, which serves as a rough
phase lock servo, includes a phase comparator circuit, which
detects the difference in phase between the reference signal
and a sweep pulse indicating the rotational phase of the
rotary heads and which has been delayed by the variable delay
of the digital phase servo. The difference in phase between
the reference signal and the variably delayed sweep pulse then
controls the output frequency of a variable frequency oscillator,
which in turn controls the phase signal used to drive a three~phase motor,
thereby controlling the rotational phase of the rotary heads.
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In this embodiment, the analog loop circuit has a
short integrator tîme constant, and good rèsponsiveness to
high frequency phase fluctuations. The analog loop circuit
functions to compensate for high frequency phase errorsr and
the digital phase servo provides accurate response to low
frequency phase errors. Thus, it is advantageous to comhine
an analog loop circuit with a digital phase servo circuit.
More particularly, there is provided:
A servo system for controlling the rotational phase
of a rotating ~ody, comprising:
reference signal source means for supplying reference
signals;
command signal source means for supplying command
signals in a predetermined phase relationship to the rotational
position of said rotating body;
phase detecting means for providing a binary signal
having only first and second discrete values in dependence upon
said reference signals being in leading and lagging phase rela-
tion, respectively, to said command signals;
signal processing means for providing successive rota-
tional phase-correction signals in a first fixed relationship
to said command signals in dependence upon one of said discrete
values of said binary signal from said phase detecting means;
means responsive to said rotational phase-correction
signals for changing the rotation phase of said rotating body;
and
means for prolonging the interval between said succ-
essive rotational phase-correction signals to a second longer
fixed relationship to said command signals in dependence upon
alterations of said binary signal between said first and said
second discrete values.
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There i5 also proyided:
A servo system for controlling the rotational phase
. of a rotating body, comprising: ~
~ reference signal source means for supplying reference ~ `
signals;
command signal source means for supplying command
~ signals in a predetermined phase relationship to the rotational
s.~ position of said rotating body;
.~` phase detecting means for providing a binary signal
` 10 in dependence upon the phase relationship between said reference
~ signals and said command signals, said phase detecting means
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.~ ~ including a first flip-flop which provides a binary signal
whenever the phase of said reference signal leads the phase of
i ~ said command signal, and ~hich provides an inverse binary signal
~ whenever the phase of said reference signal lags the phase of
~ said command signal; ~;
signal processing means for providing successive ` :~
rotational phase-correction signals in dependence upon said
binary signal from said phase detecting means, said signal
~; 20 processing means including a first AND-circuit providing an
advancing phase-correction signal upon coincidence of said
, ~ binary signal and derivative command signals provided in corres-
;;' pondence to said command signals, and a second AND-circuit ` ~:
--~ providing a retarding phase-correction signal upon coincidence :`4 of said inverse binary signal and said derivative command
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-~ means for prolonging the intervàl between said succ- ; ~.
essive rotational phase-correction signals in response to a
predetermined number of changes of said binary signal.
.~ 30 There i5 further provided:
.~ A servo system for controlling the rotational
phase of a rotating body, comprising: -~
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reference signal source mean~ for supplyi.ng reference
signals;
command signal source means for suppl~ing command
signals in a predetermined phase relationship to the rotational
position of said rotating body;
phase detecting means for providing a binary signal
in dependence upon the phase relationship between said reference
signals and said command signals, having a first state when the
phase of said re~erence signal leads the phase of said command
signal and a second state when the phase of said command signal
leads the phase of said reference signal;
signal processing means for providing succe~sive
rotational phase correction signals in dependence upon said
binary signal from said phase-detecting means;
means responsive to said rotational phase-correction
signal for changiny the rotational phase of said rotating body;
and
means for prolonging the interval bet~een said succes-
sive rotational phase-correction signals in dependence upon the
binary signal provided by said phase-detecting means, said means
for prolonging the interval bet~een successive rotational phase-
correction signals being operative in response to a change of
said binary signal from said first state to said second state
and back to said first state.
The above, and other objects, features and advantages
of the invention, will become apparent when the following
description is read in conjunction ~ith the accompanying
drawingsO
30BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram showing an embodiment of
a digital phase servo ci.rcuit according to this invention;
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Fig. 2 shows various waveforms to which reference
will be made in explaining the operation of the embodiment
shown in Fig. l; and
Fig. 3 i5 a block diagram showing a servo system
according to another embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings in detail, and initially
to Fig. 1 thereof, a digital phase servo circuit 100 according
to the invention is there shown applied to a video tape recorder
or VTR in which a magnetic tape T is scanned by rotary heads
Hl and H2 rotated by motor 1. A pulse generator P is associated
~ith the shaft connecting motor 1 to heads Hl and H2 and
generates a pulse Pb in synchronism with the rotation of heads
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Hl and H2. Pulse Pb, after being amplified by an amplfier
2, is fed to a digitally operated phase shifter or variable
delay 3. The delayed or phase shifted output of variable delay
3 is fed to a drive control circuit 4, which may be in the
form of a digital-to-analog (D-A) converter. The output of
circuit 4 is then applied to motor 1 as a driving signal for
the latter. Thus, the rotational speed of motor l depends
upon the amount of phase shifting effected by variable delay 3.
An input terminal 11 of digital phase servo 100
receives reference signals Pa which are appled to a terminal
Cp of a flip-flop 21. Another input terminal 12 receives
command signals Pb which are transmitted through an amplifier
22 to a terminal D of flip-flop 21 and to a frequency divider
or count down circuit 23. Frequency divider circuit 23 is
inoperative as a frequency divider and, therefore provides
derivative command signals Pe to first and second A~D-gates 24
and 25 for every occurrence of command signal Pb,so long as
a binary output Sd from a terminal Q of a flip-flop 17 is
low or at "O" level. If the output Sd is high or at the "1"
level, frequency divider 23 becomes operative ~o provide
derivative command signals Pe at intervals corresponding to
a succession of a predetermined number of command signals Pb.
Flip-flop 21 is shown to have output terminals Q and
Q which are connected to inputs of first and second A~D-gates 24
and 25, respectively. The output of first ~-gate 24, at which
retarding phase correction signals Sf appear, is connected to
an UP ten~nal of an up/down counter 15, and the output of second ~-gate 25,
at ~nich advancing phase correction s~onals appears, is connected to a
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D~ terminal of up/down counter 15. The digital count of
counter 15 is applied to variable delay 3, so that the latter
delays the signal provided by pulse generator P to the drive
control circuit 4 by an amount depending upon the count of
up/down counter 15.
The 3 terminal of flip-flop 21 provides a binary
signal Sc to a differentiator 18 which, in response to changes
in the level of a si~,nal Sc, provides pulse Pd to an up-counter
16. The output of up-counter 16 provides a signal to a set
terminal S of flip-flop 17 to set the latter whenever a pre-
determined number of changes in binary level Sc have occurred.
Upon setting of flip-flop 17, its O terminaf provides the
signal Sd to frequency divider 23. Therefore, after there
has been a predetermined number of changes in binary level Sc
o flip-flop 21, and until second flip-flop 17 is reset,
frequency divider 23 provides one derivative command signal
Pe of each succession of a predetermined number of command
pulses Pb
The apparatus described above with reference to
Fig. 1 operates as follows:
At the onset of a recording operation in response
to actuation of a recording button (not sho~m), a reset signal
Pr is applied to input terminal 13 and thence to the R terminals-
of up/down counter 15, up-counter 16, and second flip-flop 17,
so that counters 15 and 16 are thereby reset to zero and output
Sd of flip-flop 17 is a low level. Frequency divider 23 thus
provides a derivative command signal Pe for every occurrence
of the command signal Pb. During the recordin~ operation,
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command signal Pb applied to input terminal 12 is the pulse
provided by pulse generator P in synchronism with the rotation
of the rotary heads Hl and H2. At the initiation of a
reprcducing operation, a reset signal Pr is again applied
to input terminal 13 and command signal Pb received by
terminal 12 is the vertical synchronizing signal de-ived
from the reproduced video sig~. Reference signal Pa applied
to input terminal 11 in either a recording or reproducing
operation is provided by a clocking means (not shown) at a
rate equal to one half the standard repetition frequency of
vertical synchronizing signals of the video signals being
recorded or reproduced.
Referring to Figs. 2A and 2B, the phase of the
recording head is initially shown to be ahead of the phase of
the reference signal at time tl, and the trailing edge of the
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command pulse Pb (Fig. 2B) leads the trailing edge of reference
pulse Pa (Fig. 2A). So long as command pulse Pb returns to
zero before reference pulse Pa~ flip-flop 21 will be in the
"1" state, and Q output of flip-flop 21 will provide binary
signal Sc with a high or "1" level to AND-gate 24 and
differentiator 18. At this time tl, flip-flop 17 is stil, in
its reset stat~, and the Q output of flip-flop 17 provides a
low level or "O" signal to frequency divider 23. Thus, at time
tl~ a derivative command signal Pe is provided from divider Z3
for each occurrence of command signal Pb, as shown in Fig. 2E.
Each coincidence of "1" or high levels of signals Sc and Pe,
causes ~D-gate 24 to provide a retarding phase control signal
Pf to input terminal UP of up/down counter 15 for increasing
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the coun~ o the latter by one. ~ach change in the count
of up/do~n counter 15 causes variable delay 3 to increase
j the delay of the signal from pulse generator P by a predetermined
amount, in turn, causing control circuit 4 to change the
speed of the motor 1 so that the phase of the rotating
heads Hl and H2 is changed by an incremental amount ~, as
sho~ in Fig 2I. The foregoing phase correcting operation is
continued upon every occurrence of command signal Pb, that is,
upon every rotation of rotary heads Hl and ~, so long as the
phase of ~he rotary heads is in advance of the phase of rererence
signal Pa. Thus, in the illustrated example, during the time
tl to t2, Sc remains at the ~ t level, and a retarding phase
correction signal Pf is provided to up/down counter 15 at
each signal Pe~ The drive control circuit 4 causes the phase
of rotary heads Hl and H2 to be delayed by an additional amount
at each rotation until the phase of the rotary heads Hl
and H2 lags the phase of the reference sîgnal Pa~ as at the
time t3. At time t3, the reference signal Pa (Fig. 2A) returns
to zero before the command signal Pb (Fig. 2B) returns to zero,
flip-flop 21 changes its state for providing signals Sc (Fig 2C),
from its ~ terminal, with the binary level "O" and an inverse
binary level of "1" from its O terminal.
The coincidence OL the binary level "1" at output
; ~ terminal ~ of flip-~lop 21 with a derivative command signal
Pe from frequency divider 23 causes ~D-gate 25 to provide an
advancing phase control signal Pg ~i~. 2G) to input te~
of up/doT~n counter 15. This results in reducing the count of
up/down counter 15 by one. The change in the count of co~nter 15
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causes variable delay 3 to delay the signal to the drive
control circuit 4 by lesser amount, and as a result the phase
of the rotary heads is advanced by an amount ~.
If, for example, at the time t4, the phase of the
rotary heads is once again ahead of the phase of the reference
signal, the command signal Pb remains at "1" level after the
reference signal Pa returns to zero, and the same sequence
of operations is repeated, as at ~ime tl, so as to retard the
phase of the rotary heads Hl and H2 by an amount ~G. Once
the rotary heads Hl and H2 are brought into phase with the
reference signal Pal the digital phase servo cir.uit locks
the phase to keep it from varying ahead of or behind the reference
phase more than ~
Unfortunately, advancing and retarding the phase of
the rotary heads by ~ for successive rotations of the rotary
heads, tha~ is, every time command pulse Pb is produced, can
cause picture instability and jitter. Once the correct
rotational phase is achieved, it is beneficial to decrease the
frequency with which the phase of the rotary heads may be
^hanged. To this end frequency divider 23 acts to prolong the
interval between successive rotational phase correction signals
Pf or Pg whenever flip-flop 21 has changed its state a
predetermined number of times. In the example of Fig. 2, at
time t3 the phase of the reference signal Pa (Fig. 2A) leads
that of the command signal Pb, thus causing flip-flop 21 to
change its state for causing signal Sc to attain the level "O".
At time t4 the phase of reference signal Pa lags that of command
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signal Pb, causing flip-flop 21 to again change its state,
thereby causing binary signal Sc to return to the level "1".
Each time binary signal Sc changes its level, differentiator
18 provides up-counter 16 with a signal Ph, (Fig. 2H), and up-
counter 16, in turn, acts to set flip-flop 17 after a pre-
determined number of signals Ph are provided by differentiator
18.
Up-counter 16 can be selected so as to set flip-
flop 17 after counting any arbitrary number of signals Ph,
but that number has been chosen to be two in the illustrated
example. Therefore, at time t4, Clip-flop 21 changes state
for the second time, and differentiator 18 has provided two
signals Ph to up-counter 16, thereby causing it to set
flip-flop 17. The output terminal Q of flip-flop 17 provides
binary signals Sd to frequency divider 23 with the level "1"
tFig. 2D) and, in response thereto, frequency divider 23
provides a derivative command signal Pe only after it has
been provided with a predetermined series or number af command
signals Pb. Because derivative command signals Pe are
provided to A~-gates 24 and 25 only once for each succession
or series of a predeter~ined number of command signals Pb, for
examples, as at the time t6, the up/down counter 15, and the
phase of the rotary heads Hl and H2 is changed, only at inter-_ '
vals corresponding to a predetermined number of rotations of
the rotary head. ~he time interval between successive
rotational phase corrections is indicated as Tr on Fig. 2I.
Thus, once the rotational phase of rotary heads Hl and H2 locks
into phase with the reference signals Pa, the period between
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changes in rotational phase can be proLonged, thereby
reducing jitter and picture instability. The amount ~
by which ~he rotational phase of the rotary heads Hl and H2
is advanced or retarded, can also be made quite small in
order to avoid jitter or instability.
In the embodiment described above with reference
to Fig. 1, the command signals Pb are the input to frequency
divider 23. However, it will be apparent that the reference
signals PaJ rather than command signals Pb, may be used as
the input to frequency divider 23 in order to provide signals
Pe corresponding to a series or succession of a predetermined
number of reference signals Pa~
Fig. 3 shows another embodiment of the invention which
includes a digital phase servo 100, described above with
reference to Fig. 1, and an analog loop circuit 200 which
cooperates with servo 100 to control the rotational phase of
-rotary heads Hl and H2.
Generally, in analog loop circuit 200, reference
signals Pa are compared in phase with signals provided from
pulse generator P in synchronization with rotary heads Hl
and H2 and being passed through amplifier 2 and variable delay
3 of digital phase servo 100 on the basis of such comparison
a phase correction signal is provided to a drive circuit 45, - t
which may be a three-phase motor drive.
More particularly, reference signals Pa are shown
to be applied to a trapezoidal wave shaper 30 which provides
a succession of corresponding sloping waveforms to a sample-
and-hold circuit 41. The pulse or signal provided fram pulse generator P
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and delayed in variable delay 3 an amount depending upon the
count in up/do~ counter 15 is fed to sample-and-hold circuit
41 as the sampling pulse for the latter. The circuit 41
samples and holds, in response to the signal or pulse provided
from variable delay 3, a segment of the slope portion of the
sloping waveform from wave shaper 30. Sample-and-hold circuit
41 provides that segment of the sloping waveform to an integrator
42 which in turn provides a DC level. Such DC level is applied
to a variable frequency oscillator 43, whose output frequency
depends upon the DC 1 vel fed into it. The output of the
variable frequency oscillator 43 is then applied to a phase
modulator 44 to control the output phase of the latter provided
to drive control circuit 45 to control the phase of motor 46.
It will be apparent that, in the embodiment of Fig. 3, the
wave shaper 30 and sample-and-hold circuit 41 functicn, in
effect, as a phase comparator to compare the phase of the
reference signal Pa with the phase of the signal from delay 3.
While the embodiment of Fig. 3 shows drive control
circuit 45 to be a three-phase motor drive, and motor 46 to
be a three-phase AC motor, it is clear that a different drive
control ~eans and drive means, such as a DC control means and
motor, could be used in place of three-phase drive control 45
and motor 46.
It is clear that the analog loop circuit 200
used in conjunction with the digital phase servo circuit 100
further serves to minimize the jitter and other fluctuations
that may be caused by quantizing noise in the digital phase
servo 100.
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The descri~ed di~ital servo systems are not limited
in use to controllln~. the rotational phase of the rotary
heads of a video tape recorder, but can be used in a variety
of other servo control circuits. It is also apparent that
many modifications and variations can be made in the disclosed
embodiments of this invention by one skilled in the art
without departing from the spirit or scope of the invention
as defined by the appended claims.
