Note: Descriptions are shown in the official language in which they were submitted.
1090'~21
~.CKGROUND OF ~1-1E I1~VENTION
This invention relates generally to time dlsplacement
of time varying signals par~icularly suited to compensate
time base errors that occur in a signal reproduced from a
medium which stores the time function of the signal. When
reproducing signals stored in such a medium, ideally, the
reproduction operation is carried out under conditions iden-
tical to those existing during the record operation for the
purpose of avoiding or minimizing the introduction of unaccept-
lQ able time base errors in the reproduced time varying signal.Unfortunately, however, general time base errors can occur
frcm a variety of causes. For example, in a magnetic storage
medi~m, signal timing errors are commonly caused by changes
in medium dimensions due to environmental effects, such as
stretching of magnetic tape due to tension Rroduced by a tape
transport, head tip medium penetration changes or differences
in the relative head to medium record and reproduce velocities.
Relative head to medium velocity errors ordinarily occur as a
result of dimensional changes in the medium or imperfect oper-
~0 ation of the head motor and medium transport systems.
When recording and reproducing wideband signals con-
taining data and synchronizing information, such as picture
or video data information and periodically occurring timing or
pilot tone synchronizing information, particularly with rotary
head video tape recorders, time base errors often occur in each
period of the reproduced wideband signal. A color television
signal is a wideband signal including periodically occurring
vertical and horizontal pulses and ~ilot or color burst syn-
chronizing information and video data lnformation. Such signals
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are particularly sen~itive to ~iming errors. One form of
time base error encountered is commonly referred to as
velocity error. Such errors arise from geometric tape guiding
inaccuracies that cause Aifferences between the relative head
to tape velocity during recording and reproduction. These
errors result in a progressive phase shift of the color
subcarrier during a horizontal line. A display of a color
television signal with such phase shifts will show a progres-
sively cumulative change in color (hue) toward the right side
of the video display. This invention is particularly suited
to compensating such progressive time base errors which occur
during a period of a time varying signal.
Head to tape velocity errors comprise one component
of general time base error. Other components of time ~ase
error are the line by line mispositioning of the horizontal
synchronizing pulses (H sync) and the erroneous time or phase
relationships between components of the synchronizing informa-
tion, such as H sync and color burst. Line by line time base
errors can be compensated by correcting the phase of the
reproduced television signal line by line in accordance with
the detected position error of each horizontal synchronizing
pulse and the detected phase error of each color burst. A
known system of correcting such time base errors utilize$
adjustable time delay devices inserted in the signal path.
In such systems, the time base error is measured and the amount
of time delay inserted in the signal path is adjusted to com-
pensate for the measured error. One known system disclosed in
U.S. Patent No. 3,202,769 utilizes a vol~age variable delay
line in the signal path. A voltage corresponding to the
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measured time base error is applied to the delay line to
provide the necessary delay for the analog television
signal~ Other known systems utilize lumped element and
glass delay lines to compensate the analog color television
signal as it is transmitted along the signal path.
More recently, the utilization of digital
techniques has been proposed to provide the necessary time
base correction, One such system is disclosed in commonly
assigned Patent Application Serial No, 224,289, filed
April 10, 1975, by Maurice G. Lemoine for a TIME BASE
CORRECTOR. In this digital system, each line of the analog
television signal is converted to a digital form~ The
digitized line is then stored in a controllable digital
delay device, The delay is determined by the error signal
to provide the necessary time base correction,
Although the systems disclosed in U.S, Patent
3,202,769 and Patent Application Serial No. 224,289 correct
line by line time base errors at the beginning of each
color television line, they do not eliminate the effects
of errors progressively cumulating within each television
line or period of a signal,
Velocity error compensation has heretofore been
known, U,S, Patent No. 3,428,745 is one example of known
velocity error compensators used in conjunction with an
analog delay line time base error corrector. The time base
error corrector compensates for line by line errors at the
start of each television lineO The velocity compensator
further provides a continuous phase compensation of each
color television line to adjust chroma phase for purposes of
eliminating the progressive intra line time base error that
occurs during a iine to
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cause an undesir~ble shift of hue progressing from left to
right in the television picture. Such velocity error compen-
sators approximate the progressively cumulative error of a
line by deriving an error compensatlon signal from comparisons
of color burst phase information of widely separated lines
(64 lines in quadruplex video tape recorder) of the television
signal. This approximation technique does not provide an error
compensation signal related to the actual cumulative error of
the line whose time base is to be adjusted. Furthermore, such
compensators are unsuitable for use in conjunction with
digitized time base error correctors such as disclosed in
application Serial ~o. 224,289. Existing velocity co~sators,
moreover, vary the chroma phase by using the same vernier
delay line utilized for time base correction. The resultant
disadvantage is that the amount of velocity compensation
range is limited to the residual delay range left in the delay
line a~ter time base correction. This range may be less than
that required for complete chroma phase compensation, which
can amount to + i80 degrees of phase shift from the beginning
~ to the end of the television line.
Some time base error compensators rely on averaging
techniques to compensate for progressively cumulative time
base errors. Generally, such devices provide an error compen-
sation signal dependent upon the average of several periods of
accumulated error. Such devices are incapable of rapidly
responding to high rate and, especially, largç line to line
time base error changes.
Line by line analog delay line time base compensators
have been employed in color television signal processing systems
iO90~21
to compensaLe progresslvely cumulatlve errors in esch
line of the televiRion signal. Such existing systems
employ voltage controlled ad~ustable delay lines to
delay the analog color burst signal of one line so
that it can be phase compared to the analog color
burst of the next succeeding line. This requires a
delay line capable of providing a precise or.e line
delay. Such adlustable delay lines are very expensive
and require careful control of the electrical length
to make certain that the delayed analog color burst
is available at the correct time of the analog color
burst of the next succeeding line. An analog line by
line time base compensator of this kind is employed
in the video tape recorder manufactured by Ampex
Corporation under the model designation AVR-l.
SUMMARY OF THE INVENTION
Accordingly, the present invention relates to
apparatus for time displacing a signal while it is
being converted from a digital form to an analog form
comprising a digital to analog converter responsive
to clock signals to receive and convert a series of
digital signals to an analog representation, an
adjustable time base clock signal generator for
generating a clock signal having a time base dependent
on the time base of a time base control signal, the
cloc~ signal coupled to the digital to analog converter
to command it to convert the series of digital signals
to an analog representation, and means for generating
the tlme base control signal.
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The prcsent inventioll is advantAgeously utilized
with a system in which an ~nalog information signa] is
converted to a digital form and reconverted to an analog
form and particularly when the analog signal includes
periodically occurring timing or synchronizing signals.
A preferred embodiment of the device is designed to be
utilized in conJunction with a digital time base
corrector, such as disclosed in the aforementioned
application Serial No. 224,289 to compensate for time
base errors line by line. In that use, the reproduced
analog information signal is converted to a digital
form, whereupon it passes through a digital delay
network and is reconverted to an analog form.
As utilized in a color television recorder, the
velocity compensator includes circuitry for continuously
providing a representation of the color burst phase of
each line of the television signal and comparing the
phase of such continuously provided color burst phase
representation with the phase of the color burst of
the next occurring or succeeding television line. The
detected phase difference is representative of the
amount of color hue or chroma phase change or time base
error that occurs during the television line between
successive color bursts and concomitant correction
required in that line. Means are provided to generate
a line by line error signal representative of the
detected phase difference~ This error signal is
coupled to control the time base of an ad~ustable clock
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1(~90~21
.
generator whereby the ~ime base of the clock signal generated
thereby can be independently determined for each line. The
clock signal generated is utilized by a digital to analog
converter to reconvert the digitized television information
to ~n analog form with the progressive chroma phase error
compensated. Advantageously, the progressive phase error is
compensated by progressively varying the clock signal rate
accordingly. The digital to analog converter thusly is
operated to provide storage for holding the signal until
clocked out in analog compensated form.
To generate the error signal, the velocity compensator
continuously provides a representation of the color burst
phase of each television line so that the phase of the color
burst may be compared with the phase of the color burst of a
succeeding line. An error signal is derived from the compari-
son and used to compensate time base errors in the line of
television information between the successive color bursts.
If color burst signals are not available from successive
television lines in time to permit the determination and
correction of errors in the television line occurring between
the bursts before the reconversion of the di~itized line of
television information to analog form, the time displacement
adjustment device of the present invention contemplates the
provision of a suitable delay device for the digitized tele-
vision information to afford the necessary time delay.
The time displacement device of the present invention
conveniently provides very accurate timing displacement of
time varying signals because it determines the desired time
displacement by the periodic examination of a continuously
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,
available representation of the actual timing or synchroni-
~ing component contained in the signal and it independcntly
generates a separate time displacement signal for the period
between successive examinations. Each separate, independently
~enerated time displacement signal provides very accurate
time displacement of the defined period and the provision of
successive time displacement signals enables very accurate
continuous timing adjustment of the time varying signal.
DESCRIPTION OF A PREFERRED EMBODIMENT
The compensator of the present invention is described
in detail in the following with reference to the sole figure
illustrating one preferred embodiment of the invention as
arranged to compensate velocity errors in a color television
signal reproduced from a magnetic recording thereof by a rotary
head magnetic tape recorder. However, it will be readily
appar~nt upon consideration of the description of SUCil preferred
embodiment together with the foregoing that the present inven-
tion can be utilized to introduce time displacements in other
signals for purposes other than correcting time base errors,
such as for the purpose of intentionally time displacing error-
free signals. As long as the signal being displaced in time
includes a synchronizing component, such as the color burst
signal in a composite color television signal or a continuously
occurring pilot tone in other information signals, providing
signal time base information, the time displacement device of
the present invention can be utilized to determine and effect
a desired alteration of the signal time base. Furthermore,
the time base of time varying signals provided by sources other
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than magnetic tape or other recorders can be advantageously
altered in a controlled fashion by use of the time
displacement device of the present invention.
Referring to the sole figure, a reproduced analog
color television signal received from the rotary head of a
magnetic recorder is applied to the input of a time base
corrector 10, wherein the misposition of the horizontal
synchronizing pulses and erroneous phase of the color burst
are corrected at the beginning of each horizontal line, A
lC digital time base corrector advantageously employed with
this velocity compensator embodiment of the present invention
includes an analog to digital converter and is more
particularly disclosed in the aforementioned Application
Serial No. 224,289, The time base corrector 10 digitizes
the color television signal, which then travels along a
main path through a digital two line delay 20, A delay of
two nominal television lines is required for operation of
this embodiment of the velocity compensator for reasons to
become apparent, The time base corrected and delayed
digitized signal is reconverted to an analog form and
output by digital to analog converter 30, The rate of
conversion by converter 30 is controlled for a purpose to
be described by a line by line adjusted clock signal applied
thereto from variable clock generator 50, Because the time
base correction process usually renders the input
sychronizing components preceding the video component of each
line unuseable, the corrected video information output by the
converter 30 is passed through a standard video singal
processor (not shown), which inserts new synchronizing
information to reconstitute a compensated color television
signal.
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T~le color burst signal present at the beginning of
each television line is detected, sampled and digitized by
the corrector 10, the digitized sample of the burst signal
being applied to a recyclable burst store 40 for storage and
continuous availability during the remainder of the
television line. Because the digital time base corrector 10
described in the above mentioned application serial no,
224,289 includes a memory circuit that stores and
continuously regenerates in analog form an actual
representation of the color burst signal included in each
television line for the interval of the television line,
this velocity compensator embodiment of the present
invention may conveniently utilize the device as recyclable
burst store 40, The memory circuit employed as recyclable
burst store 40 receives a sample of the color burst in
digital form and stores the sampled burst for the entire
length of the television line so that its timing
relationship to the line of information is retained. This
is accomplished by storing and repetitively regenerating
the color burst sample in a recyclable register clocked
by a signal derived from the fixed timing reference signal
provided by the reference subcarrier source 49, The burst
store 40 includes a digital to analog converter (not shown)
wherein the continuously regenerated sample of the burst
signal is reconverted to analog form so that it may be
utilized in the phase comparison circuitry of the velocity
compensator to generate a representation of the time base
of the burst signal continuously at least during a portion
of the next line of the television signal including its
color burst signal, A more detailed description of this
burst store 40 can be had by reference to aforementioned U.S.
Application Serial No, 224,289,
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The analog burst signal ~epresentation is applied
by burst store ~n throuyh a zero error switch 52 to.the inputs
of analog burst hold 54 and of velocity error measure 64.
Zero error switch 52 i5 schematically indicated in the figure
as a simple switch but may comprise any suitable circuitry to
switch between two positions or sta~es. In the normal position
as shown, the burst signal is applied to the analog burst hold
54 and velocity error measure 64, each of which has a phase
lock loop for generating and holding a representation of the
],o phase of the applied burst signal.
The phase lock loop of analog burst hold 54 consists
of phase detector 56, sample and hold 58 and phase shifter 60.
Phase detector 56 compares the phase difference between two
signals applied at its inputs and provides a difference signal
at its output as a dc voltage level which is coupled to the
input of the sample and hold 58. The output of the sample and
hold 58 is coupled to the control input of the phase shifter 60.
Phase shifter 60 is controlled by the sampled difference signal
provided by the sample and hold 58 to shift the phase of a
signal applied at its input in accordance with the difference
signal. In this embodiment, a reference signal having a fixed
time base is provided by a fixed frequency reference subcarrier
so~roe 49 to the clock input of the phase shifter. me reference
signal is shifted in phase before being applied from the output
of phase shifter to one input of phase detector 56. The analog
burst signal obtained from the television signal being corrected
is coupled directly to the other input of phase detector 56,
which gen,erates a difference signal in a known manner having a
value necessary,when applied to phase shifter 60,to maintain the
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phase of the two signals applied thereto coincident. Th~
phase of the applied reference subcarrier is shifted in
accordance with the sampled difference signal to coincide with
the phase of the color burst applied to phase detector 56 ?
In this manner, a representation of the phase of the sampled
color burst of each line of the input television signal is
obtained.
The shift in the phase of the applied reference sub-
carrier occurs, hence, phase representation,is obtained during
a 32 usec. sample period just prior to the end of the televi-
sion line including the color burst then being generated by
burst store 40. The length of the sample period is made
sufficient to assure accurate operation of the analog burst
hold 54. At the end of the 32 usec. sample period, the phase
loak loop opens at the input side of sample and hold 58,
whereby the dc voltage difference signal provided by the phase
detector 56 at the end of the television line to be corrected
is held at the output of the sample and hold to maintain the
shifted phase reference subcarrier representation of the color
burst phase beyond the end of that line.
The foregoing sample and hold operations are con-
trolled by ga~ing signals provided by a timing generator 41.
Timi~g generator 41 includes digital counters interconnected
by ~ogic circuits, which respond to the occurrence of each of
the horizontal sync (H sync) and vertical sync (V sync) pulses
provided by a reference source and signal H sync provided by
the time base corrector 10 at inPut terminals 42a and 42b and
42c, ~espectively, to generate the required, properly timed
gating signals at the output terminals 43. The 32 usec. sample
1~)909Zl
period is d~fincd by a 32 usec. gating signal present at the
output terminal 43a of the timing genera~ox 41. Thi3 gating
si~nal is yenerated during a 32 usec. period at the end of
the period of a television line. In the NTSC standard, the
television line period is approximately 63.5 usec. To correct
NTSC color television signals, the timing generator 41 is
arranged to issue the 32 usec. gating signal about 31.5 usec.
after the occurrence of the signal H sync at its input terminal
42c.
The 32 usec. gating signal is coupled to the sampling
control input 59 of sample and hold 58. During the presence
of the gating signal, the sample and hold 58 is enabled to
sample the voltage level of the difference signal then being
generated by the phase detector 56. A voltage storage means,
such as a capacitor, is included in the sample and hold 58 for
storing a voltage representative of the sampled difference
signal. At the end of the 32 usec. gating signal, the sample
and hold 58 is disabled, thereby terminating the sampling of
the difference signal and opening the phase lock loop. However,
the sample and hold 58 stores the voltage representation of the
sampled difference signal existing at the end of the 32 usec.
sample period until the occurrence of the next 32 usec. gating
signal generated during the succeeding television line. Since
the phase shifter 60 receives this voltage representation until
changed, it retains and continuously makes available the phase
or intra line timing of the signal, iOe., color burst applied
to analog burst hold 54 beyond the end of the television line
in which the color burst occurredO Because the recyclable
register of the burst store 40 provides a representation of the
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actual analog burst si~nal obtained from the beginning o~
the television line to be corrected, the phase shifted
reference subcarrier provided by the phase shifter will
contain the actual phase or intra line time base information
from the beginning of the television line, As will be
explained in detail below, this signal and another
containing the actual phase or intra line time base
information at the end of the television line being
corrected are used together to correct the actual velocity
errors that accumulate during that television line.
Velocity error measure 64 is also a phase lock
loop comprised of phase detector 66, sample and hold 68 and
phase shifter 70, As in analog burst hold 54, a
representation of a color burst signal from zero error switch
52 is applied to one input of phase detector 66 of the
velocity error measure 64. The other input of the phase
detector 66 is coupled to receive the phase adjusted signal
continuously provided by the phase shifter 70. For reasons
that will become more apparent from the detailed
description hereinbelow of the operation of phase shifter 70,
the sample period here, however, occurs in each television
line for 27 usec, during the occurrence of the signal
color burst provided by the recyclable burst store 40
through switch 52 and prior to the 32 usec. sample period
of the same line. The 27 usec. sample period is defined by
a 27 usec. gating signal generated by the timing generator
41 in response to the signal H sync and issued at its output
terminal 43b. The length of the sample period is chosen
for convenience in operation of the circuit. Because the
length of a television line of an NTSC singal is
approximately 63.5 usec,, the 27 usec., and 32 usec,
Sample periods do not overlap. Since the actual phase
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of a bu~st siynal is held and continuously provided by analog
burst hold 5~ beyond the end of the televislon line in which
the burst signal occurs, its phase information is available
at the output of phase shifter 60 during the 27 usec. sample
period of the succeeding line. It can be readily seen that
the actual phase of the color burst of a first line is
available in analog burst hold 54 to be compared with the
actual phase of the color burst of a second succeeding line
by velocity error measure 64.
The operations of the phase detector, sample and
hold, and phase shifter elements of velocity error measure
64 are like those of the analog burst hold 54, except for
timing and actual signals received and generated. The phase
adjusted or shifted output signal provided by the phase shifter
lS 60 of the analog burst hold 54 is used as the clock for phase
shifter 70. Unt`il the 27 usec. gating signal is received by
sample and hold 68, the phase of the output signal from phase
shifter 70, which is applied to phase detector 66, will coincide
with the phase of the output of phase shifter 60. However,
the phase detector 66 generates an error signal ~7henever a
phase difference exists between its input signals. During the
27 usec. sample period the phase lock loop of velocity error
measure 64 is closed by the 27 usec. gating signal provided by
timing generator 41 to the sampling control input 69 of the
sample and hold 68. Any error signal present at the output of
the phase detector 66 is sampled and stored by the sample and
hold 68. To sample and store the phase difference signal
generated by the phase detector 66, sample and hold 68 is
arranged to respond to its gating signal in the same manner as
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the sample and hold 58 of analog burst hold 54. The error
signal provided by sample and hold 68 is provided as a dc
voltage level representative of the difference in actual phase
between the first and succeeding color bursts. This error
signal causes the output phase of phase shifter 70 to shift
to coincide with the phase of the color burst of the later or
succeeding television line applied to phase detector 66. At
the end of the 27 usec. sample period the loop opens at the
sample and hold in the manner described with respect to sample
and hold 58 and the error signal is held at the output of
sample and hold 68 until the next 27 usec. sample period.
The held error signal or dc voltage level represents the
magnitude of the actual cumulative phase error for the hori-
zontal line period associated with the color burst whose phase
or intra line time base information is being held in analog
burst hold 54. Because of the cooperation of the velocity
error measure 64 and analog burst hold 54, the cumulative
phase error information is continuously available for one line
beyond that in which it occurred.
The output from sample and hold 68 is connected to
the inputs of two error hold circuits 72, 74 connected in
parallel to one another. The error signal generated by
velocity error measure 64 is sampled by the error hold circuits
72, 74 for 20 usec. immediately after the velocity error
measure 27 usec. sample period is terminated. The error holds
72, 74 are commanded to sample the error signal by a 20 usec.
gating signal provided by the timing generator 41 at its output
terminal 43c. This gating signal is coupled to the sampling
control inputs 73 and 75 o~ the error holds 72 and 74 respectively.
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.
The 20 usec. ga~ing signal causes one error hold to store a
voltage xepresentative of the voltage error signal then being
provided by the sample and hold 68 of the velocity error
measure 64.
The outputs of error hold circuits 72, 74 are con-
nected respectively to two inputs of error switcher 76,
schematically represented by two switches as shown, which
alternately applies the error stored in either error hold
72, 74 to the input of a line by line error hold 78 one line
at a time. Error switcher 76 is controlled by timing generator
41 to change states every horizontal line so as to alternately
connect error hold 72 and error hold 74 to line by line error
hold 78, which is likewise controlled by the timing generator
to sample and hold the error signal coupled thereto by error
switcher 76. The error switcher 76 is switched at, and the line
by line error hold 78 is enabled for a period of 8 usec.
following,the occurrence of reference H sync. Timing generator
41 generates an 8 usec. gating signal at terminal 43d to be
applied to error switcher 76 at control terminal 77 and to
error hold 78 at sampling control input 79 once each horizontal
line. While the error stored in one error hold (e.g., error
hold 72) is being applied to line by line error hold 78 by
error switcher 76, the other error hold (e.g., 74) is ready
(having applied the error stored therein during the previous
line) to receive and store the error generated by velocity
error measure 64 for another television line. An error hold
is unable to receive and store an error signal present at its
input until its contents have been transferred to the line by
line error hold 76. Thus, error holds 72, 74 alternate in
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storing ~hc error for every other television line and linc
~y line error hold 78 alternately stores the error signal
stored in error hold 72 and error hold 74. Both error holds
72 and 74 are required since the error signal for each
television line must be determined as that line is reproduce,d
by the recorder and may be available for storage before the
error for the previous line has been sampled by line by line
error hold 78.
For 8 usec. following the occurrence oÇ the reference
H sync at terminal 42a of timing generator 41, line by line
error hold 78, which is a sample and hold circuit, is commanded
by the 8 usec. gating signal to sample the velocity error
applied to its input as explained. During the remaining
period of the line, the velocity error is held by the line by
line error hold for processing by ramp generator 80.
Ramp generator 80 produces a variable slope sawtooth
waveform. It includes a controlled positive current source 82
and a controlled negative current source 84, both connected
to charge a capaçitor 86. The rate of current flow produced
by the two current sources in conjunction is controlled by
the voltage level of error signal stored by line by line error
hold 78 and applied to ramp generator 80. The output of error
hold 78 is coupled to the control input of positive current
source 82. The current drawn by negative source 84 is normally
constant and continuous. A ramp is generated for the length
of at least the video information portion of one television
line, having a slope determined by the current level generated
by the sources and, hence, by the voltage level of the error
signal and, therefore, proportional to the actual velocity error
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of one televi~ion line. At the end of each television line,
which has a prec~etermincd ~ITSC standard length of 63.5 usec.,
ramp reset 88 recei~es at its control input 89 the ~ usec.
gating signal provided by timing generator 41. The ramp
reset 88 operates in response to that gating signal to reset
the ramp generator to nominal zero hy discharging capacitor
86 to ground. Simultaneously, the error value for the next
television line is sampled and held by line by line error
hold 78 and then applied to positive source 82 of ramp
generator 80 to generate a new ramp having the requisite slope.
Ramp generator 80 operates to generate a new ramp for the
length of the video information portion of a television line
following each 8 usec. sample period of line by line error
hold 78.
The variable sawtooth waveform thereby generated is
applied to output phase shifter 90. This phase shifter is
linear and similar to phase shifters 60 and 70 in structure
and operation. A reference clock, which may be the same as
provided by reference source 49, is applied to the input 99
of output phase shifter 90 to provide the basic signal fre-
quency. The phase shifter is controlled, however, by the
sawtooth waveform applied to the control input of the phase
shifter to linearly shift the phase of or time displace the
clock signal gradually across each television line.
The phase shifted signal is applied to an adjustable
clock generator 50 to generate a clock signal whose time base
can be varied by shifting it in phase by an amount dependent
upon the actual phase shift represented by the signal provided
thereto by phase shifter 90. The clock generator 50 includes
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a pulse shaper and frequency multiplier that cooperate to
shape the signal applied to its input from-the output of
phase shifter 90 and multiply the frequency to produce a
clock signal suitable for use by digital to analog converter 30.
S Clock generator 50 is coupled to digital to analog
converter 30 to control the timing rate of conversion of the
television signal from digital to analog form in a known
manner. For small progressively cumulative intra line time
base errors converter 30 operates to store temporarily the
1~ signal in digital form. The temporary storage is achieved
by converting the digitized signal to analog form at a timing
rate controlled by the time base of clock 50. Since the rate
of conversion of the television signal from digital to analog
form by converter 30 is controlled by the time base adjusted
clock signal, which is applied thereto in a known manner, the
phase of the television signal will be linearly expanded or
monotonically
compressed/across each television line in accordance with the
determined actual velocity error of that line. Since, as
previously explained, the velocity error is determinèd by the
erroneous phase difference between representations of actual
color bursts of two successive television lines, the shift in
hue due to velocity error between the beginning and end of
each line is known. The shift is presumed to be linear across
each line.
If it is desirable to maintain a highly calibrated
system, ramp generator 80 is arranged to determine and maintain
a nominal zero reference. This is accomplished during each
vertical blanking interval between television fields. During
this period for a time of five lines, zero error switch is
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switched to replace at the input to the analoy burst hold 54
the ou~put from burst store 40 with the reference subcarrier
signal provided by studio reference 49. This is controlled
by a signal from timing generator ~1 applied from terminal
43e to control terminal 53 of zero error switch 52. The
same control signal is applied to terminal 93 to activate
zero error sample and hold 92 for a purpose to be described.
Under this condition reference subcarrier is applied to both
phase detector 56 and phase shifter 60. There is no phase
difference and the output from phase shifter 60 is the unshifted
reference subcarrier signal. The reference subcarrier signal
is also applied to phase detector 66. Velocity error measure
64, therefore, detects no phase difference and the error signal
applied to ramp generator 80 represents zero velocity error.
When activated, zero error sample and hold 92 samples this value
and utilizes this zero value to calibrate the ramp generator
negative current sources 84 which is a controlled current source.
The input of zero error sample and hold is connected to capacitor
86 so that a voltage representation of the slope of the ramp
generated in response to a zero phase shift issampled and held.
It is thereafter applied by the output of zero error sample and
hold to the control terminal of negative current source 84 to
set the current level to the proper rate to calibrate the ramp
generator to nominal zero which, in ~his embodiment, constitutes
a zero slope ramp.
The velocity error compensator of the present invention
provides chroma phase correction with a full range of + 180 degrees
without the use of glass or vernier delay lin~s, thereby elimina-
ting the consequent expense and limited range thereof. The true
spirit and scope of the invention will become evident by reference
to the following claimsO
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