Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
In general, this invention relates to techniques of
altering the time-base of time varying signals. More particu-
larly, however, it concerns a time altering technique especially
suited for electronically correcting undesirable time-base
differences in time varying signals.
BACKGROUND OF THE INVENTION
During the processing of time varying electrical
signals for signal transformation, analysis or correction,
( :
frequently, the time-base of the signal must be altered or
compensated. For example, signal time-base compensation is
commonly employed to correct~undesirable time-base differences
in signals having recurrent time-base synchronizing components.
Alteration of a signal time-base to correct undesirable time-
lS~ base differences is particularly important when the signalundergoes transformations between different domains, such as
occur in recording an~d reproducing signals on magnetic or other
forms of record media. During the recording and reproduction
~ processes, the time function of the signal is transformed into
;~ 20 a space function and then back into the time function. As the
~ signal undergoes the transformations, timing or time-base errors
:~ are often intxoduced to the signal. The dynamic or time variant
class of such time-base errors prevents the achievement of the
necessary transient-free and time-stable signal reproduction
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required in high resolution signal processing systems. For
example, time-stable signal generation is desirable in all
television signal processing systems and highly stable generation
mandatory in systems used to prepare television signals for
public transmission. ,~
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Two techniques are employed to correct undesirable
time-base errors in signals reproduced from a record medium;
electro-mechanical and electronic. Electro-mechanical
techniques are employed to correct gross time-base errors and
achieve such correction by synchronizing the operation of the
signal recording and reproducing equipment. Electronic
techniques are employed to correct smaller residual time-base
errors not corrected by the electro-mechanical devices and
achieve such correction by time displacing the signal after
its reproduction. It is the electronic technique of time-base
error correction to which the present invention is relevant
Heretofore, electronic signal time-base alteration
systems have employed adjustable time delay devices inserted
in the signal path to correct time-base errors. In such
systems, the time-base error is measured and the amounk of time
delay inserted in the signal path adjusted to compensate for
and, thereby, correct the measured time-base error. One
particular type of system which enjoys widespread use has a
voltage variable delay line in which lumped constant inductors
and voltage variahle capacitive diodes are interconnected in a
delay line configuration. A voltage, corresponding to the
measured time-base error~ is applied to the ~ariabl~ d~odes to
fix the necessary delay for correcting the time-base error.
A description of a voltage variable delay line type signal
time-base alteration system can be had by reference to U. S.
Patent No. 3,202,769.
In another type of electronic signal time-base
al~eration system, a number of fixed delay lines, or a single
delay line with a series of taps spaced therealong, are arranged
in combination by electronic switches. Time-base errors are
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corrected by operatin~ the switches in accordance with the
measured error to selectively insert the necessary corrective
delay in the signal path. A fixed delay line type signal ti~.e-
base alteration system is described in U. S. Patent No. 3,763,317
and a tapped delay line type signal time-base alteration system
is described in U. S. Patent No. 3,7~8,-36-8-.-
Recently, digital delay devices, such as clockedstorage registers, have been used in systems for correcting
time-base errors in analog signals. In the digital systems,
the analog signal being corrected is di,gitized, corrected and
reconstituted. Correction is performed hy entering or writing
:
the digitized signal in an adjustable storage register at a
fixed rate determined by the frequency of a reference clock
signal. ~he storage register is operated to correct time-base
errors by reading the signal from the register at an adjusted
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faster or slower rate, depending upon the time-base error.
This technique of constant write rate and variable read rate
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cannot handle large discontinuous or incremental time-base
changes in the .signal. I~ ma~netic tape recorders, such
; 20 incremental time-base changes are commonly caused by anomalies
their operation and most commonly when switching hetween
maynetic transducer heads.
~ ~ In sign~l time-hase alteration systems, especiall~
;~ those arranged to eliminate time-base errors and provide a high
degree of signal time-base stability, it has been the practice
to cascade coarse time-base correction device~ and fine time-
base correction devices. Voltage variable delay line systems
have been used to provide the desired fine time-base correction
while switched delay line systems have been used to ~rovide the
coarser time-base corrections. Because all such delav line
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systems are analog devices, they are prone to drift and have
other disadvantages characteristic of analog devices. Incre-
mental time-base changes that occur as a result of anomalies
in the operation of tape recorders often cause errors or costly
interruptions in the performance of signal processing operations
because of the inability of these time-base error correction
devices to respond to the incremental changes. Also, if a large
range OI time-base errors is required to be corrected, large
and complex correction systéms are necessary.
Considerable advantage is therefore to be gained by
utilizing a technique to perform signal time-base compensation
that is able to af~e~t all time-base alterations, including
incremental, without error. Additional advantages will be
realized in the performance of such signal time-base compensa-
lS tion by first altering the signal time-base by any fraction of
a known increment required to bring the signal within an intesra1
number of known increments of the desired time-base reference
~ and, thereafter, alterlng~the signal time-base by such integral
; numher of known increment to adjust the signal to the desired
time base.
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SUMM~RY OF THE I~JENTION
A feature of this invention ls the utilization of
digital techniques to alter signal time-base which enable
digital circuits to be employed ~hat are far less expensive
; ~ 25 to construct and maintain than analog circuits. Another feature
of this invention is that time-base compensation can be per-
formed withQut the need of an analog measurement of the amount
of compensation desired, thereby avoiding all of the disadvan-
tages characteristic o~ analog measurement circuitry. It is
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yet another feature of this invention to re-time the signal
by a fraction of a known increment by temporarily storing the
signal in a time buffer at a time adiusted in accGrdance wi.h
the desired time-base change while maintaining the storage
retrieval time fixed relative to an established time-base
reference. A f~ ther eature of this invention is that further
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incremental alterations in the time-base of a signal can be
- performed without error by adjusting the further storage
retrieval time of the signal in accordance with a desired
time-base change while maintaining the storage entry time
fixed relative to an established time-base reference. Still
another feature o~ this invention i5 that alterations in the
time-~ase of a signaI grester than one principal division of
the time-base, as determined by the period of one cycle of
the signal's time-base component, can be performed by first
altering the signal time-base by any desired amount correspond-
ing to a fraction of the princ}pal time-base division and
thereafter further incrementally altering the signal time-~ase
by any desired~amount corxes~ondlDg to an integral number of
principal time-base div~ SiOIIS.~ Yet another feature of this
invention is that time-base alterations are performed hy the
use of a derived con rol signal which reduces the effect of
noise to a large degree. These and other features of this
invention provide particular advantages when the invention
is ernployed to eliminate time-base errors in television
signals reproduced ~rom video recording equipment.
; In accordance with this invention, an information
signal whose time-base is to be altered, i.e., compensated, is
- sam~led to obtain representations oE the si~nal. The informa-
tion signal must contain or be provlded with a time-base
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component, appearing at least at intervals of the information
signal. A timing or time-base reference, such as a clock
signal having a frequency that remains stable relative to
the nominal frequency of the time-base component associated
with the uncompensated information signal, is initially
employed to control the`sampling time and rate. The
reference clock signal must be generated relative to the
occurrence of the information signal so that at least a
portion of the information signal's time-base component is
sampled a number of times. Such sampling must be sufficient
to permit regeneration of the time-base component from the
representations thereof.
As the time-base component is sampled under the
control of the stable reference clock signal, the represen-
tative samples are stored and, thereafter, used to regenerate
; a representation of the time-base component, which is
frequency stable relative to and phase coherent with the
original time-base component associated with the uncompen-
sated information signal. An information clock signal is
derived from the regenerated time-base component so that its
frequenc~ and phase characteristics are stable relative to
those of the regenerated, hence, original time-base component
associated~ with the information signal. During the interval
of the information signal following the portion of the
time-base component from which the information clock signal
is derived, the derived information clock signal is used to
time or control additional processing of the information
signal for the introduction of the desired amount of time-
base alteration.
The use of a derived clock signal obtained in the
above described manner provides particular advantages in the
further processing of an information signal, such as, for
example, a television signal, to alteriits time-base for the
purpose
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OI eliminating timing differences or time-base errors that
commonly occur ir, such signals. ~en employing the techni~ue
of this in~ention to eliminate time-base errors, that occur in
the television signal, the frequency and phase of the reference
clock signal is maintained fixed and the derived clock signal
is employed to time the fur-ther sampling of the informaiion
si~nal during the interval following the portion of the infor-
mation signal's time-base component rrom -~hich the information
clock signal is derived. To eliminate time-base errors from
color television signals, the information clock signal is
derived from a re~enexation of the color synchronizing burst
that occurs at the beginning of each horizontal line in-terval
of the composi~e color television signal. The thusly derived
clock signal is employed to time the sampling of the video
information signal component following the synchronizing interval
located at the beginning of each horizontal line of the tele-
vision signal.
~ Following the further sampling, the obtained repre-
- sentations of the video si~nal a e written in a clock isolator
;20 or time buffer at times determined by the derived clock signal.
Thereafter, the video signal representations are read ~rom the
buffer at a time deterrnined by the fi~T~frequency and phase
5 `!~
reference clock signal. Tn this fashion, the time buffer
serves to re-time the video signal representations relative to
the reference clock signal. The oriyinal form of the video
signal may be reconsti-tu~e~ from the re-timed sampled representa-
tions read from the buffer.
'rhe use of a clock signal derived from a reyeneration
of the time-base component of an information signal to time the
further processing or sampling of the inormation signal is one
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of the fund~mental features of this invention that facilitates
the alteration of signal time-base. As described hereinabove,
the derivation of the information clock signal in this manner
assures that the frequency and phase of the derived clock signal
will always be precisely related to those of the time-base
component contained in the information signal. Therefore, the
time-base of the derived clock signal will follow changes in
- the time-base relationship of the information signal and timing
reference. Because the tlme-bas~ of the derived clock signal
is precisely locked to that of the information signal and the
derived clock signal is used to control the further sampling
of the information signal, the information signal will always
be further sampled at the same points during its interval
regardless of the time-base relationship of the information
signal and timing reference. Changes in the time-base
; relationship of the information signal and timing reference
- will not change the sample points during the information
signal interval. This enables the thusly sampled information
signal to be re-timed relative to any desired time-base refer-
e~ce, regardless of changes in the time-base relationship of
the information signal and timing reference. As will become
readily apparent upon consideration of the following detailed
descriptions of preferred embodiments of the signal time-base
alteration technique of this invention, the derivation and use
of the information clock signal to further sample the informa-
tion signal enables outstanding advantages to be realiæed in
the implementation of the technique, the most significant of
which is the precise time-base error corrections of television
signals with a high degree of reliability.
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Ordinarily, the time-base component of an in crmation
signal is a simple periodic signal. However, some information
signals, such as television signals, have several time-base
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components arranged to define principal~and sub-periods of the
information signal and intra-period time-base conditions thereof.
Because such time-base components have different frequencies,
it is possible in some circumstances for sub-periods to appear
properly aligned relative to a reference even though the higher
ordered periods are not properly aligned. To avoid the possible
harmful effects that could be caused by a false indication of
proper time-base alignment, the highest frequency time-base
:
component is selected for deriving the information clock signal.
Signal time-base compensation up to one cycle of the highest
frequency time-base component is automatically prov1ded by the
15; above described technique of using the derived information
clock signa] to further sample the information siynal. If
signal time-base compensations greater than one cycle of the
highest frequency time-base component are necessary to achieve
the proper time-base alignmer1t, the information s1gnal is
further examined to dekermine the numher of full cycles it must
further be altered to properly align its time-base. Tne required
further alteration is accomplished by storing the s~mpled
representations in a memory for a number of cycles correspond-
ing to -th~ determination. Preferably, the further alteration
is performed after the sampled representations have passed
through the time buffer.
In addition to altering the time-base of an information
signal to eliminate undesirable time-base differences, the signal
time-base compensation in accordance ~Jith this invention can be
employed to introduce wanted time-base chan~es in an information
2~
signal. Such wanted time-base changes are introduced by al~er-
ing the time-base of the reference clock signal in accordance
with the want~d time-base changes. In other respects, the
signal time-base compensation of this invention is performed
as described above with reference to the elimination of time-
base errors. ~ltering the time-base of reference cloc~ signal
causes a change ln the time-base relationship of the reference
clock signal and time-hase component contained in the informa-
tion signal. As previously explained, such relative
; 10 time-base change introduces a comparahle time-base difference
between~the time-base of the sampling of the information signal
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~ and that of the time-base altered reference clock signal.
:
Therefore, reading the samples of the information signal from
the time buffer~ at times determined by the time-base altered
reference clock signal results in the re-timing of the infor-
:
~ mation signal relative to the altered reference signal and,
:: - : ,
thereby, the introduction of the wanted time-base changes in
the information slgnal.
As will be appreciated~from the foregoing, signal
20~ time-base compensation in accordance with the present invention
is easily adaptable to digitalization and, therefore, is able
to benefit from the advantages that can be gained by the use
of digital circuits. Furthermore, the ability to alter the
time-base of an information signal first by a fractiQn of a kno~n
time increment or principal time-basa division and, thereafter
- by any amount equal to an integral number of such increments,
régardless of the size of the time-base alteration, has the
advantage of avoiding the limitations associated with
~ascading analog time-base alteration devices.
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BRIEF DESCR-PTION F T~.'P, DRAWIN~S
The ~oregoing as well as other featurGs and aavan-
tages of the signal time-base alteration technique of this
invention will become more apparent upon the consideration of
the following detailed description and claims toyether ~,~7ith
the accompan~ing drawings o~ which:
Figure 1 is a block diagram of a digital time-base
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compensator in accordance with this invention adapted for a
color television signal;
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Figure 2 is a detailed block diagram illustrating
the construction of the recyclable digital store of the
compensator of Figure 1;
.
Fiqures 3A and 3B are timing diagrams illustrating
the operation of the signal time-base~compensation in accordance
with this invention in elimlnating time-base errors from color
television si.gnals;~
Figure 4 illustrates circuits in block form that
permit the time-base compensator of Figure 1 to correct errors
:; greater than one cycle of the signal's color synchronizing
: 2G burst.
Figure 5 illustrates circuits in block form that
permit the time-base compensator embodiments of Figures 1 and
4 to operate when the incoming signal is a monochrome television
signal.
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DESCRIPTION OF PREFERRED EMBODIMENTS
The signal time-base compensator 110 ~ accordance
with the present invention is shown in Figure 1 as arranged
to eliminate time-base errors present in a color television
information signal reproduced by a video recorder (not
shown), such as a magnetic disc recorder. However, it will
be appreciated that the principles of this invention are
equally applicable for performing other signal time-base
compensations, such as correcti~g time-base errors present
in other time varying information signals, eIiminating
differences in reIative time-bases of signals and purposely
altering the time-base of signals. With particular reference
to Figure 1, the uncorrected color television signal repro-
duced ~y the disc recorder is applied to the input of an
analog-to-digital,,(A/D), c~nverter 111,, which is operable to
provide at its output 112 an encoded signal in the form of
a pulse code modulated representation of the television
signal. This representation signal is further processed
to be eventually coupled error-free to a digital-to-analog
(DjA) converter 113, which'decodes the digitized signal and
reconstitutes at an output 114 the television signal in
analoy form. Because the synchronizing components included
in the television signal issued by the D/A converter 113
usually are misshaped and contain undesirable transients as
a result of their passage through the compensator 110, the
television signal is coupled to an output processor 116 of
the type commonly used in video recorders. Such processors
116 operate to strip the synchronizing components from the
incoming television signal and insert new properly shaped
and timed synchronizing components in~o the signal to form
the desired composite television signal at its output 117.
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In the compensator 110 of the invention, the encoding
A/D converter 111 provides a multi-bit word representation of
the incoming signal at output 112 each time the converter 111
is clocked by a clocking signal applied over a line 118, as
shown. The converter 111 is clocked to sample the instantaneous
analog amplitude of ~he l.ncoming televi.sion signal, such that
a succession of binary words is developed at its output 112,
each word consisting of a number of binary bits, which bits
together represent a particular amplitude level in a binary
: 10 format. In general, this operation of analog-to-digital con-
version may be referred to as pulse code modulation of the
incoming signal. The reverse of this operation is performed
by the decoding D/A converter 113. The decoding converter 113
receives the binary encoded words at ~n input coupled to line llg
IS and, in response to a succession of reference clock signals received
over lines ]21 and 122, issues a reconstituted or decoded analog
television signal to an output processor 116, which co~nunicates
the corrected televlsion signal to the output 117. In accor-
dance with this invention, the time-base error compensation is
20~ achieved by deriving a clock signal from a time-base component
included in the ielevision signal so that the clock time of
the derived clock signal is coherent with the time-base
component. The derived clock ~ignal is employed to clock the
A/D converter 111 to sample the uncorrected television signal
and effect the encoding of the television signal lnto the
digi~al b.inary ~ord representations. After encoding, the
d.igitized television signal is time buffered and decoded ~t tne
~/A co~verter 113 by a clock signal at a clock time coherent
with a reference time-base si.gnal, such as a reference color
subcarrier. As a result of such buffering and decoding, the
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decoded television signal is rendered in-phase with the
reference color subcarrier.
In the case of a color television signal, precise
time-base corrections can be achieved by deriving the infor-
mation-signal-related clock signal from the color synchronizing burs-
time-base component located on the back porch of each horizontal line
blanking interval. The derivat:ion i.s achieved by coupling to
~:~ the input of a recyclable digital store 123 binary word
representations of one or more cycles of the signal's color
; 10 burst available at output 112 of the A/D converter 111. The
store 123 provides a digital memory for a plurality
of binary words corresponding to the amplitude levels of the
signal's color burst at sample times~ By storing the binary
words available durlng the sampling of the signal's
15. color burst, suf~icient information is memorized in
s~ore 123 for repetitively regenerating a full cycle of the
color burst such that a continuous signal identical to the
uncorrected television si~nal's color burst can be developed
lasting beyond the durati~on o~ ~the signal's color burst. The
derived clock signal is obtai.ned b~ further processing the
continuously regenerated color burst signal and is employed to
digitize the remainder of the horizontal line of the television
signal from which~it is regenerated.
To insure that the continuous signal, hence, derived
clock signal regenerated from the c~lor burst samples stored
in the rec~clable store 123 remains in-phase with the color
burst, hence, uncorrected tele~ision signal, the A/~ converter
111 i5 first clooked during ~he sampling of the television
. . .
;~ signal's cu'or burst and storing of the resulting samples by a
- 30 clock signal a~ a clock time coherent with the reference clock
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,, , ,, , , , , , , , , , , , , , . ,, . . . , ... . _ . . . _ . . . .
`~ signal. Thus, the A/D converter 111 must be clocked b~ two
clock control signals over line 118. The initial clocking
occurs during a sampling and storing mode, preferably,
lasting for several cycles of the color burst time-base
component. During this initial mode, the clock input (CL)
of A/D converter 111 receives over line 118 a clock control
signal locked in-phase to the reference clock signal. The
A/D converter 111 is clocked by the second, derived clock
control signal re eived over line 118 during a following
recycling mode, which lasts for the remainder of the hori-
zontal line interval after the initial clocking. For these
two modes of operation, a switching means generally indicated
at 124 is provided having a switching device 126 disposed
in a first or sampling and storing state connecting the line
118 to the clock output line 122 from a ~3 reference clock
source 128. Switching device I26 is actuable to a second
or recycling state, which connects line 118 to the derived
clock signal provided by a digital memory circuit 129 over
line 127. In the recycling mode, switchingdevice 126 con-
nects the clock input (CL) of the A/D converter 111 with aX3 signal clock 13I providing a clock output for memory
circuit 129. The X3 signal clock 13I is responsive through
a bandpass filter 132 to an output of a D/A converter 133.
The D/A converter 133 converts or reconstitutes the binary
word representations of the signal color burst recycled in
the recyclable store 123 into an analog form. Accordingly,
the signal available from thé D/A converter 133 appears as
a continuous unfiltered replica of the input signal time-
base component, which, in this preferred embodiment, is a
sinusoidal color burst of a television signal. The bandpass
filter 132 is set to provlde a center frequency equal to
that of the color burst of the signal being corrected, which
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in the case of NTSC standardized color television signal is
a frequency of 3.58 megahertz. Filter 132 in its location
between the output of D/A converter 133 and an input to X3
signal clock 131 has been found to provide an advantageous
restoration of the color burst frequency following the
various conversion and digital storage manipulations. If
a number of signal color burst cycles are sampled and stored
in store 123 for regenerating the derived clock signal, the
filter 132 will average any noise contained in the recycled
signal color burst over the number of stored cycles, thereby
improving the timing accuracy of the derived clock signal.
As indicated above, switching device 126 of -
switching means 124 is normally in its illustrated second
;or recycllng state, connecting X3 signal clock 131 to the
clock input (CL) of the A/D converter 111 so as to control
the sampling and time the encoding of the uncorrected
teIevision signal with the rec~cled color burst samples
derived from the signal. To provide for the actuation of
switching device 126 to its other first or sampling and
storing state, switching means 124 includes circuitry for
detecting the occurrence of the color burst time-base
component in the television signal and responsively operating
device 126 in accordance therewith~ In particular, a sync
separator 134 ~s provided for detecting at the input of the
compensa~or 11~ the occurrence of each horizontal sync pulse
(SIG H) appearlng during the blanking interval of each
horizontal line of the television signal. The output of
-the separator is coupled to the input of a switch control
pulse generator 136. Upon the detection of the leading
edge of the horizontal sync pulse, the separator 134 issues
a command to the pulse generator 136.
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After an interval o~ approximately 6 microseconds, the pulse
generator 136 issues a pulse lasting about 2.0 microseconds
for actuating the switching device 12~ to its sampling and
storing state. Thus, in response to the appearance of a
horizontal sync pulse at the input to the A/D converter 111,
separator 134 and pulse generator 136 cause switching device
126 to apply the encoding X3 reference clock signal to the
clock input (CL) of the converter 111, which responsively
digitizes a selected number of cycles of the signal's color
burst. The timing of the operations of the separator 134
and pulse generator 136, as specified herein, is arranged for
NTSC television signals so that the switching device 126 is
actuated to its sampling and storing state during the mi~ddle
interval of the color burst interval. It is dasired to
~15 arrange the sampling and storing of digital representations
of the signal's color burst to occur in the middle of the color
burst interval because this interval is the most accurate and
reliable in representation of the color synchronizing burst
frequency. In addition, the derivation of the information-
signal-related clock signal is less susceptible to errors that
may be introduced by small changes in the location of the color
burst on the back porch of the horizontal blanking interval.
To condition the recyclable store 123 to store five
cycles of the color burst digital representations, a burst
detector 137 is connected to the input of the compensator 110.
Upon the occurrence of the aolor burst in the incoming televi-
sion signal, the burst detector 137 issues command on line 138,
which extends to the write enable input (WE) of the recyclable
digital store. This command causes the store 123 to write the
multi-bit binary words appearing at output 112 from the A/D
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converter 111. The actual writing or storage operation
occurs at each reference clock time determined by a clock
signal input to storage 123 from X3 reference clock 128.
The ensuing operation of recyclable store 123 may be best
described with reference to both Figures 1 and 2.
With reference to Figure 2, store 123, includes a
random access memory 139 having conventional write and address
control inputs, as indicated by (W) and (A) symbols respectively.
~ A binary word input is connected for receiving the multi-bit
binary word at output 112 of the A/D converter 111. A binary
word output is provided for issuing the recycled digital
~signals to line 140. An address generator 141 is responsive
to a source of X3 reference clocking signals over line 122
and provides over a connection 142 address signals for write
and read access to memory 139 in accordance with the generated
address signal. Included within store 123 is a write clock
generator 143 responsive to the command received over line 138
from burst detector 137. The command sets the write clock
generator 143 to issue over line 144 write enable signals to
the writ enable input (W) of the random access memory 139 each
time a X3 refe~ence clock is received from line 122. As long
as write enable signals are received by the random access
memory 139, the binary words issued by the A/D convertar 111
will be written for storage in the memory 139. The store 123
also includes a counter 145 responsive to the command received
at its reset (R) input coupled to line 138 from burst detector
137. The command resets the counter 145 for counting addresses
issued by the address generator 141. The counter 145 is also
reset by an internally generated command as will be described
below. Each time the counter 145 is reset, it issues a reset
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command oYer line 146. The first reset command issued
following the command provided over line 138 by the burst
detector 137 is coupled to disable the previously enabled
write clock generator 143 by resetting it until the next
command is issued by the burst detector 137. In this manner,
the random access memory 139 is prevented from receiving
further binary word representations o the television signal
after fifteen samples of color burst have been received.
The counter 145 also serves to recycle the address generator
ld 141. Each time the address generator 141 issues an address
signal, the enabled counter 145 is clocked by a X3 reference
clock signal received from line 122 to examine via a line 147
the address issued by the address generator 141 and coupled to
its data (D) input. When the counter 145 detects ~he issuance
1 of the last of fifteen address signals issued by the aadress
generator 141, it issues a reset command to the address generator
over line 146. The counter also uses this reset command
internally to reset itself to again examine address signals
issued by the address generator 141. In this manner, the
address generator 141 is continuously cycled through the fifteen
addresses identifying the locations in the random access memory
139 in which the fifteen multi-bit binary words representing
the five sampled cycles of signal color burst are stored.
A further explanation of the operation of the recyclable store
123 will be provided herein with a description of an actual
operating sequence of the compensator 110.
In selecting the rate at which the incoming informa-
tion signal must be sampled, the clocking or sampling requency
must be at least two times the maximum signal ~requency which
the system is to pass without substantial degradation.
_~_
Furthermore, the clocking rate and storage capacity of the rando~.
access memor~ 139 must be selected such that the number of digitized
samples stored in the random access memory 139 is equivalent to an
integral number of full cy~les of the time-base component of the
S signal, iOe., equal to the product of the number of samples per cycle
or period of the time-base component and an integral number of the
cycles. With the clocking rate and storage capacity thusly selected,
the random access memory 139.carries an integral number of digital
representations of full cycles of the timing component of ~he
signal, which when recycled results in the generation of a con-
: tinuous clock signal during the recycling mode. In the case of a
color television signal, both:the storage capacity and the sampling
~: rate criteria are advantageously satisfied by selecting the encoding
clock signal to have a frequency three times the color burst fre-
quency:and by storing fifteen samples of the. color burst. Accord-
ingly, i.n the exemplary embodiment, X3 signal clock 131 includes a
frequency mu~ltiplier for multlplylng by~a~factor of three the con-
tinuously regenerated color burst signal developed by s~-ore 123,
~ D/A converter 133 and th~ bandpass fil~er 132. It is observed tha~
:: 20: the frequen~y o the encod.ing clock signal employed during the
. sampling and storing mode must he nominally equal to the established
encoain~ rate, although the phase~may differ from the dPrived clock
signal in accordance with the time-base error of the signal .~eing
:~
: ~ compensated.
In the embodiment of ~igure 1, the basic reference
time-base signal is the referance color subcarriar available~
for example, from the studio reference source for synchronizing
~ all of the studio equipment for broadcast purposes. This
: reference color subc~rr.ier is applied to a refer~nce signal
processor 148 which is a conventional component providing for
compensation o fixed delays existing in cables and the like,
--~3.--
~, "~
,,
... . . . . . . . . . . ... . . . . . .
2Z
and for developing the necessary phase alteration of the
reference signal for European color systems, such as PAL
~phase alternating line). The output of the processor 148
provides the basic reference time-base signal relative to
S which the compensator 110 operates to compensate the incoming
television signal. Because of the need of a X3 reference clock
signal, the frequency of the basic reference time-base signal
is multiplied by a factor o three by a frequency multiplier
included in the X3 re~erence clock source or generator 128.
: ~
~ 10 Since a Xl reference clock signal is required by the most
,
~ preferred form of the compensator 110, a Xl reference clock
:
generator 149 is coupled to receive the reference time-base
signal from the processor 148 and provides over line 121 the
required Xl re~erence clock signal.
In accordance with the foregoing selection of
encoding and decoding clock rates, the A/D converter 111
functions to develop a separate binary word at each of the
three clock times occurring during the period equal to one
cycle of the color burst. In ~his instance, A/D converter 111
is designed to provide an 8-bit word at each clock time, with
these 8 bits providing a 0 to 256 amplitude level capacity for
the digital representation of the incoming television signal.
Recyclable digital store 123, therefore, has a 15-word capacity,
again with each word consisting of 8-bits. As there are three
sampling points for each cycle of the color burst, the random
access memory 139 of the store 123 provides for storing five
full cycles of the digitally represented color burst. In
operation, while the pulse generator 136 issues the 2 microsecond
pulse in response to the detection of the horizontal sync pulse,
the memory 139 is commanded by write clock generator 143 (upon
.~ ;;-, ~3
, ~,
the occurrence of burst) to write or store thé binary words
occurring at output 112 of thé A/D converter 111 at thé -
instant of each X3're~erenced clock signal received over
line 122. With reference to Figure 2, this operation in
particular provides for address generator 141 accessing a
new word store in memory 139 in response to each of the X3
reference clock pulses, each newly accessed word store
receiving the instantaneous bit conditions of the binary
word at output 112. The 2 microsecond pulse'issued by the
pulse generator 136 temporarily sets the switching device
126 in its sampling and storing state, thereby coupling
the X3 reference cloc~ signal to clock the A/D converter
;~ 111.
After the five cycles of the digitized color
burst have been stored the storing operation is terminated
by the counter 145 detecting via line 147 the fifteenth
address generated by the address generator 141 following
the issuance of the 2 microsecond pulse and issuing the
reset command to the write'cIock generator 143. The reset
command disables the write clock generator, théreby removing
write enable signals from the random access memory 139.
Following the termination of the sampling and
storing mode, the'address generator 141 continues to access
memory 139 in response'to the X3 reference clocking signal
over line 122, repeating in sequence'the same fifteen word
locations accessed during the write operation. This causes
the stored 8-bit words to be successively read out over
output line 140 to the D/A converter 133. The memory 139
is permanently disposed in an active read mode, such that
the stored binary words are:'continuously read out over line
140. The read function is operational during the storage
of new digital information received from ...
-Z4-
the A/D converter 111 by the operation of a by-pass switch
151. The switch 151 has two inputs and one output. One input
of the by-pass switch 151 is connected by line 153 to the output
of the random access memory 139 and the other input is connected
by the by~pass line 154 to the line 112 at the input of the
store 123. While set to provide write enable signals during
the sampling and storing mode, the write clock generator 143
conditions the by-pass switch 151 to connect lines 112 and
140, thereby, passing directly to the output the binary words
being stored in the memory 139. At the end of the sampling
and storing mode, the write clock generator 143 is disabled,
hence, placing the switch 151 in a co,ndition to couple output
- line 153 of the memory 139 to the line 140. The switch 151
remains in this condition during the entire recycling mode,
enabling the s~ored color burst words to be coupled to the
D/A converter 133 for derivation of the information-signal-
related clock signal. The provision of the by-pass switch 151
enables tha X3 clock signal circuits to be readied for the
generation of derived X3 clock signal.
During the recycling mode, the address generator 141
and counter 145 function together to cause the repetitive
generation of the same address sequence. This results in the
binary words stored in the memory 139 being rep~titively read
in such sequence throughout the remaining duration of the
horizontal line i~terval following the color burst.
Figures 3A and 3B illustrate the manner in which the
derived clock signal is generated to be in-phase with the
time-base component of the information signal from which it
is derived. Figure 3A illustrates the condition that would
exist if the incoming color television signal was without error.
,. ~ ~S
.. . .
41~;~2
. .
.
, .. ~ "
During the sampling and storing interval, the X3 reference
clock causes the sampling of the signal's color burst in the
A/D converter 111 and the storing of the sample values in the
recyclable store 123. Because the incoming television signal
is without error, the first sample of each cycle of the signal's
color burst occurs at the beginning of the color burst cycle.
Upon the recycling of the fifteen words stored in store 123,
the output of the filter 132 will be in-phase with the color
burst contained in the incoming television signal If a time-
base error exists in the incoming television signal, asillustrated by Figure 3B, the sample values represented by the
binary words ob~.ained from the ~/D converter 111 will be
: ::
different~ This difference exists because of the time-base
difference between the reference time-base signal and the
incoming television signal, hence, the differ~nt sample
,
points during the color burst cycle. ~pon recycling the
fifteen words stored in store 123, the regenerated color burst
signal output by the filter 132 will be in-phase with the
color burst contained in the incoming television signal
:~ :
Hence, tne signal clock derived from the filter output will
alway~ be in-phase with the time-base component contained in
the television si.gnal regardless of time-base changes or
errors that may occur therein.
~hile in this instance a random access memory,
~ 25 address generator and counter means have been employed for
; ~ ~ recyclable .store 123, it will be appreciated that other
digital storage circuitry may be used ln place thereof.
For exa~ple, a recycling shift register is capable of providing
the function of store 123, as will be recognized by those
3~ skilled in the art.
,~'
zz
To simplify the avoidance of errors in the re-timing
of the digital repr~sentations of the television signal output by
the A,/D CGnVerter 111 during the recycling mode, a time buffer 156
is employed having a l-word serial to 3-~ord parallel converter 157
at its input and a complementary 3-word parallel to l-word serial
converter 158 at its output. The converters 157 and 158 are shown
in Figure 4. The succession of individual binary words developed
at output 112 are passed into the serial-in-parallel-out converter
157. This conver-ter 157 receives each of the succession of binary
. ~ ~
words at a clock rate of ~ times;the recycled signal color burst
by applying ~he clock pulses from the X3 clock sources available on
line 118 to the clock (Chj input of this converter as indicated.
The converter 157 is constructed to store three of the binary words
generated at output. 112 ln~a serial fashion and is of the kind
15~ wherein each new word added to the~converter shifts the last word
out leaving the converter always loaded with three full binary words.
The~serlally loaded;informatlon~is~transferred ln parallel fashion
:: ~: : : .
to the converter 158 through a clock isolator 163 (See Figure 4)
inc~luded in the time buffer 156. During each line interval o the
20~ înput television~signal, the transfer time to the clock isolator 153
occurs at the clock time de~ermined by clock pulses developed hy a
lX signal clock 159 ~S~ee Figure l). The lX signal clock is connected
to the output of bandpass filter 132 so as to generate a cloc~ pulse
signal at the recycled color burst rate, which is the rate of the
~25~ color burst as it occurs at the heg-nning of the line interval.
In particular, the lX signal clock 159 is provided by limiting the
filter output and using a positive going leading edge of the thereby
gen~.ated square waveform to provide the clock pulses.
Each positive goiny leadincl edge of the limited regenerated
color burst identifies the beginning of ~ cycle of
Z
the color burst. The lX signal clock 159 is connected to the
time buffer 156 over a line 161. In this manner, the clock
isolator 163 receives in response to each applied clock pulse
the full contents of the conYerter 157, which as discussed
above carries at all times three full binary words generated
by the A/D converter 111 at output 112. Moreover, the three
words received in a parallel format by the clock isolator 163
correspond to the three words developed during one cycle of
the regenerated color burst.
The output of the converter 157 is a 24-bit word
coupled to the input of the clock isolator 163. The isolator
is able to simultaneously read and write the 24-bit words.
Because the isolator 163 is able to read and write simultaneously,
the clocking operations can occur on the input and output sides
lS thereof with reference to different ~ = clock signals,
~hereby providing time buffering and the ability to re-time
signals. To write or store the output of the converter 157,
clock signals generated by signal clock 159 are coupled by
line 161 to write address (WA) and write enable IWE) inputs of
the isolator 163. This clock signal is in-phase with the
color burst of the uncorrected television signal. The stored
24-bit words associated with each cycle of the time-base
component are read or output from the isolator 163 in response
to lX reference clock signals provided by a reference clock
generator 149 and coupled to a read address (RA) input of the
isolator 163 over line 121.
By clocking the isolator 163 with the two clock
signals, the phase of output of the isolator will be re-timed
and synchronized to the reference color subcarrier phase.
_~ _
2Z
Converter 158 is the complement of converter 157 in
that it provides a parallel-in-serial-out transfer of the
digital word infor~ation received from converter 157 through
clock isolator 163. Converter 158 thus reconverts the dlgital
5 information to a l-word serial format, however, in this
instance the serial words are clockecl out of the converter 158
at a clock time determined by the lX reference clock applied
to converter 158 over line 121, as indicated in Figure 4.
These serial words are applied over line 119 to the input of
the D/A converter 113 and, thereupon, decoded under the control
of the 3X reference clock present on line 122. The D/A
converter 113 reconstitutes the desired analog signal at output 114
synchronized to the reference subcarrier phase.
In the foregoincl manner, the digital compensator of
this invention functions to synchronize an qncoming information
; signal with a reference or standard~time-base signal. It is
observed that the range of time correction is, in the present
embodiment, a period~correspon~ing to ~ full cycle of the
time-base component. More particularly, in the case of a color
:~ :
television signal, the correction range is one cycle of the
colcr burst~frequency which~is one divided by 3.58 megahertz
; or approximately .28 microseconds. If the phase error of the
incoming ~ si~gnal is likely to exceed this range, such as
may occur when reproducing television signals from tape
recorders, then the signal issued at output 114 will be shifted
50 ~S to synchronize the phase of the color burst component
to the reference color subcarrier. ~Iowever, the horizontal
sync of the television signal will be improperly phased relative
to the reference horizontal sync signal. For certain applica-
tions, such as in conjunction with disk recording equipment,
~r~, `''~.' .
.
.
the correction range o one full cycle of color burst, or0.28 microseconds providea by this embbdiment, is adequate
without the aid of additional time-base error compensating
systems.
If larger time-base errors are likely to be
present, a random access memory 164 is inserted between the
clock isolator 163 and the parallel-to-serial word conver-
ter 158, as shown in Figure 4. The memory 164 corrects the
time-base of the signal by increments equal integral whole
numbers of the period of one cyale of color burst. This is
accomplished by writing the 24-bit word at addresses in the
memory 164 determined by a write'address generator 166.
The memory 164 is enabled a~ its enable input (WE) to write
the 24-bit word and the generator 166 is clocked by the lX
reference~clock on line 121. The' contents of the memory
164 is read according to the address provided by a read
address generator 167. The read address supplied by genera-
~; tor 167 is determined by the relative time of the
occurrences of the hbrizontal sync pulses of the incoming
signal and of the refer'ence.'~The'relative time'ofoccurrences is determined by a counter serving as a
horizontal sync comparator 168. The counter 168 is started
to count in response to the reference horizontal sync and
is stopped by the occurrence of the television signal's
horizontal sync. The counter 168 counts at the rate of
color burst. The output of the counter 168 is coupled to
the set (S) input of the read address generator 167 and
changes by setting the output read address in accordance
with the number in the counter 168 following the occurrence
of the television signal's horizontal sync.
The successive 24-bit words are written at sequen-
tial addresses of the' memory 164. The capacity of the
memory 164 .....
-30-
22
can be adjusted as desired. For a correction of at least one
horizontal line inter~al, i.e., about 53.5 microseconds, the
memory 164 is arranged to have a capacity of 256 words. Each
word reprasents a time of one period of color burst, i.e.,
about 0.28 microseconds. Therefore, a capacity of 256 words
will provide in excess of 63.5 microseconds of storage. The
read address generator 167 is set relative to write address
generator 166 so that if the signal horizontal sync and
reference horizontal sync are in phase~ identical addresses
generated by the two generators will be separated in time
equivalent to that required to cycled about one-half the
capacity of the memory, with the write address generation in
advance of the read address generation. For a one horizontal
line interval correction capacity, the separation is about
32 microseconds.
.
The foregoing construction and operation of this
invention applies to a system for correcting an information
signal having a recurrent time-base synchronizing component
in the form of a burst of alternating amplitude variations,
such as color burst. ~This invention is also capable of time-
base error compensa~ion of information signals lacking or
having time-base components in a form other than an alternating
amplitude time-base signal. For example, a monochrome tele-
vision signal may be corrected in accordance with the principles
of the present invention by inserting an artificial burst or
pilot signal consisting of a burst of alternating amplitude
variations into the television signal during a blanking
interval thereof. In particular, such a burst signal may be
added to the back porch of each blanking interval accompanying
a horizontal line of the monochrome television signal, wherain
--~32--
"~
2Z
the harizontal sync pulse serves as the time-base component
to which the inserted pilot signal is selected to have a
predetermined phase relationship.
With reference to Figure 5, a modification of the
system of Figure 1 is illustrated for compensating a monochrome
television signal by -inserting an artificial burst signal
consisting of a burst of alternating amplitude time-base
~ .
information. Burst insertion is provided by a ringing
oscillator burst generator 171 having an input controlled by
10~ the uncorrected monoahrome horizontal sync provided by the
sync separator 134. An output line 173 of generator 171 is
~; provided for issuing a burst of alternating amplitude time-base
information for insertion into the monochrome television
; signal at a summing junction 174 by a lead 177 from a gate
lS 176. Junction 174 is provided by a conventional signal sum-
ming circuit. By this arrangement the generated artificial
:
burst signal is inserted in the monochrome television signal
prior to application of the incoming signal to the encoding
A/D converter 111, in this instance. Such arrangement is
- 20 operable only by the absence o a color burst occurring in the
incoming signal. To this end, a connection is made from the
output of the burst detector 137 to gate 176 to disable the
gate whenever a color burst is detected in the incoming signal.
Apart from the fact that in the system of Figure 5
the burst signal is artificially generated and inserted, this
system for use with monochrome television signals functions in
substantially the same manner as ~ ~n~-r~n~-~e~Son~~th
the system of Figure 1 used for color television signals.
The artificial burst generator 171 is designed so as to generate
a burst signal having the same frequency and phase relationship
3~
22
as a color burst, so that the standard reference color
subcarrier may be employed as the reference time-base signal
in the monochrome circuit of Figure 5. This is achieved in
accordance with the present invention by generator 171
receiving fxom sync separator 134 the horizontal sync pulse
of each monochrome television line as it appears in the incoming
television signal and~employing the leading edge of the
;~ horizontal sync pulse to trigger a phase controlled ringing
circuit designed to provide a f~equency of oscillation e~ual
to that of the standard color burstj which in turn is nominally
equal to the frequency of the reference color subcarrier.
The phase of the output burst signal generated by ringing
generator 171 is controlled in accordance with the output of
a divide by 2 flip-flop 179 having an input responsive to the
leading edge of the horizontal sync pulse as developed by
sync separator 134. The flip-flop 179 has a pair of outputs
- 181 and 182 corresponding to oppoæite sides of flip-flop 179,
; thus issuing signals which are 180 opposed. The purpose of
divide by 2 flip-flop 179 is to drive phase controlled ringing
20 ~ oscillator 171 such that it develops a 180 phase change at
each television line so as to conform the artificially generated
burst signal to the standard phase alternation existing between
color burst and sync timing in a NTSC standardized color
television signal.
Accordingly, flip-flop 179 responds to each
horizontal sync pulse by changing states. In response to a
first horizontal sync~received from separator 134, output 181
will switch from a low to a high state while output 182 will
simultaneously switch from a high to a low state. The next
horizontal sync pulse will cause an opposite transition.
~!! 3 ~
~L4~1~Z2
Phase controlled ringing oscillator 171 is designed to respond
only to output transitions from outputs 181 and 182 exhibiting
a low to high change in state.
As each artificial burst appears at output 173
following the horizontal sync pulse, the 2 microsecond pulse
output provided by the pulse generator 136 actuates the
gate 176 by disposing it in its set condition. Also a mono/
color switch 183 is set to couple the pulse from the pulse
~ .
generator 136 to control the recyclable store 123 in pIace
of the burst detector 137.
~ .
::
