Note: Descriptions are shown in the official language in which they were submitted.
FIELD OF THE INVFNTI0~
In general, this lnvention relates to techniques
of altering the time-base of time varying signals. More
particularly, 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 compensa-
tion is commonly employed to correct undesirable time-base
differences in signals having recurrent time-base ,
synchroni~ing components. ~lteration of a signal time-
base to correct undesirable time-base differences is
particularly important when the signal undergoes -
transformations between different domains, such as occur ~`
in recording and 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 a space function and then back into
the time function. As the signal undergoes the trans-
formations, timing or time-base errors are often
introduced 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 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.
Two techniques are employed to correct undesirable
time-base errors in signals reproduced from a record
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medium; electro-mechanical and electronic~ Electro-
mechanical techniques are employed to *orrect gross time~
base errors and achieve such correction by synchronizing
the operation of the signal recording and xeproducing
equipment, Electronic techniques are employed to correc~
smaller residual time-hase errors not corrected by the electro-
mechanical devices and achieve such correction by time dis-
placing 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 amount of time
delay inserted in the signal path adjusted to compensate for
and, thereby, correct the measured time-base error. One
particular type o system which enjoys widespread use has a
voltage variable delay line in which lumpea constan~ inductors
and voltage ~ariable capaciti~e diodes are interconnected in a
delay line configuration. A voltage, corresponding to the
measured time-base error, i5 applied to the variable capacitive
diodes 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 ~o
.
U.S. Patent No 3,202,769 issuea August ~4, 1965 to Columbia
Broadcasting System, Inc.
In another type of electronic signal time-base
alteratlon 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
erroxs are corrected by operating the switches in accordance
with the measured error to selectively insert the necesC;ary
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correcti~e delay in the. signal path. ~ fixed delay line type si~nal
tim~-base alteration system is described in U.S. Patent No. 3,763,317
issued October 2, 1973 to ~r~x Corporation and a tapped ~elay line type
si~nal tIme-base alteration system is described in U.S. Patent ~D.
3,748,366 issued ~uly 24, 1973 to Ampex Corp~ration.
Recently, digital delay devices, such as clocked s~orage
registers/ have ~een used in syste~s for correcting time-base. erro~s in
analog signals. In the digital systems, the analoy si~nal being oorrected
.
is digitized, corrected and reconstituted~ Correction is performed by
entering or writing ~he digitized sign21 in an adjustable storage register
at a fixed rate determined by the frequency o~ a reference clock signal.
The storage regis~er is operated to correct time-base errors by reading
the signal from -~he register at an adjusted ~aster or slower xate, depen~ung
upon the ti~e~base error. This tec~ique of constant write rate and
variable read rate cannot handle large discontinuous or incremental time-
base changes in the signal7 I~ magne-tic tape reoorders, such incremental
tin~ ase changes are c~mmonly caused by anomalies in their operation and
most commonly when switching between magnetic transducer heads.
In signal $ime-base alteration sys-tems, especially those
alIe~ged to eliDin. te time-base errors and pro~ide a high degree of signkal
time-base stability, it has been the practice to cascade coarse time-bHse
correction devices and fine time-base correc~ion devices. ~olt~ge variable
dela~ line systems have been used to provide the desired fine t~me-base
correction while swibched delay line systems have been used ~o proviae
the coarser time-~ase correcti~ns. Because all such delay line systems
are analog devices, they are prone to drift and have other disadva~ages
charactRris~ic of analog devlces.
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Incremental time-base changes that occur as a result of
anomalies in the operation of tape recorders often cause
errors or costly interruptions in the perfornance of
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signal processing operations because of the inability of !
these time-base error correction devices to respond to
the incremental changes. A]so, if a large range of
time-base errors is required to be corrected, large -
and complex correction systems are necessary.
Considerable advantage is therefore to be gained
by utili~ing a technique to perform signal time-base
compensation that is able to effect all time-base
alterations, includ:Lng incremental, without error.
Additional advantages will be realized in the performance
of such signal time-base compensation by first altering
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the signal time-base by any fraction of a known increment
required to bring the signal within an integral number I
of known increments of the desired time-base reference
and, thereafter, altering the signal time-base by such
integral number of known increment to adjust the signal -
to the desired time-base.
SUMMARY OF THE INVENTION
.:
A feature of this invention is the utiliæation
of digital techniques to alter signal time-base which ,-
enable digital circuits to be employed that are far less
expensive to construct and maintain than analog circuits. ~ ;
Another feature of this invention is that time-base
compensation can be performed without the need of an
analog measurement of the amount of compensation desired,
thereby avoiding all of the disadvantages characteristic ;
of analog measurement circuitry.
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In accordance with this invention there is
provided apparatus for altering the time-base of an
information signal relative to a reference signal
defining a known time-base, the information signal
including a time-base component of a known nominal
frequency. The apparatus comprises means for receiving ~
and storing each of successive intervals of the information '
signal for a time corresponding to a fraction of one
cycle of the nominal frequency; and means for receiving ~;
each successive lnterval of the stored information
signal and further storing it for a time corresponding ~ ~;
to an intergral number of cycles of the nominal frequency.
In the particular embodiments described, as
the time-base component is sampled under the control of
the stable reference clock signal, the representative
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
uncompensated information signal. An information clock
signal is derived from the regenerated time-base component
so that its frequency 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.
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The IlSe 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 alter its `
time-base for the purpose of eliminating timing
differences or time-base errors that commonly occur in
such signals. When employing the technique of this
invention 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 s employed to time the further
sampling of the information signal during the interval
following the portion of the inEormation signal's
time-base component from which the inEormation clock
sl~nal is derived. To eliminate time-base errors from
color television signals, the information clock signal
is derived from a regeneration of the color synchronizing
burst that occurs at the beginning of each horizontal
line interval of the composite color television signal.
The thusly derived clock signal is employed to time the
sampling of the video information signal component ~ ~;
following the synchronizing in~erval located at the ;
beginning of each horizontal line of the television
signal. ~ ~;
Following the further sampling, the obtained ~ ;
representations of the video signal are written in a
clock isolator or time buffer at times determined by
the derived clock signal. Thereafter, the video signal
representations are read from the buffer at a time
determined by the stable frequency and phase referenc~e ~ `
clock signal. In this fashion, the time buffer serves
to re-time the video signal representations relative to
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the reference clock signal. The original form of the
video signal may be reconstituted from the re-timed
sampled representations read from the buffer.
The use of a clock signal derived from a
regeneration of the time-base component of an
information signal to time the f~lrther processing or
sampling of the information signal is one of the
features of the apparatus that facilitates the alteration
of signal time-base. As described hereinaboveS 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 tho~se 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 time-base 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 reference, 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
detai]ed descriptions of preferred embodiments of the
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signa] time-base alteration technique of this invention,
the derivation and use of the information clock signal ~
to further sample the information signal enables ;
outstanding advantages to be realized in the implementa-
tion of the technique, the most significant of which
is the precise time-base error corrections of television `
signals with a high degree of reliabil;ty.
Ordinarily, the time-base component of an
information signal is a simple periodic signal. However,
some information signals, such as television signals,
have several time-base components arranged to define
principal periods 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 provided by the above described technique
of using the derived information clock signal to further
sample the information signal. If signal time-base `
compensations greater than one cycle of the highest
frequency time-base component are necessary to achieve
the proper time-base alignment, the information signal is
- 30 further examined to determine the number of full cycles ~;~
it must further be altered to properly align its time-
base. The required further alteration is accomplished
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by storing the sampled representations in a memory 3
for a number of cycles corresponding to the determination.
Preferably, the further alteration i8 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 with this invention can be employed to ~
introduce wanted time-base changes in an information ;
signal. Such wanted time-base changes are introduced
by altering the time-base of the reference clock signal
in accordance with the wanted 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. Altering
the time-base of reference clock signal causes a change
in the time-base relationship of the reference clock
signal and time-base component contained in the information
signal. As previously explained, such relative time-base
change introduces a comparable time-base difference
between the time-base of the sampling of the information
signal 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 information signal relative to the
altered reference signal and, thereby, the introduction
of the wanted time-base changes in the information signal. ,
As will be appreciated from the foregoing, signal
time-base compensation in accordance with the present
invention is easily adaptable to digitalization and,
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therefore, i9 able to benefit from the advantages that
can be gained by thc llse of digital circuits. F-lrther-
more, the ability to alter the time-base of an
information signal first by a fraction of a known time ~
increment or principal time-base division and, thereafter ~ '
by any amount equal to an integral number of such `~ ~:
increments, regardless of the size of the time-base
alteration, has the advantage of avoiding the limitations
associated with cascading analog time-base al.eration
10 aevices. -~
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BRIEF DESCRIPTION OF THE DRAI~INGS ::
The foregoing as well as other features and -~
advantages of the signal time-base alteration technique
of this invention will become more apparent upon the
consideration of the following detalled description
and claims together with the accompanying drawings of
which:
Figure 1 is a block diagram of a digital time-
base compensator in accordance with this invention
adapted for a color television signal;
Figure 2 is a detailed block diagram illustrating :
the construction of the recyclable digital store of the
compensator o~ Figure l; :
~ igures 3A ~nd 3B are timing diagrams
illustrating the operation of the signal time-base
compensation in accordance with this invention in j:
eliminating time-base errors from color television
signals;
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 synchroni~ing burst.
Figure 5 illustrates circuits in block orm
that permit the time-base compensator embodiments of
Figures 1 and 4 to operate when the incoming signal is
a monochrome television signal.
DESCRIPTION OF PREFERRED EM~ODIMENTS
The signal time-base compensator 110 in
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
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magnetic disc recorder. ~lowever, it will be appreciated
that the principles oE this invention are equally
applicable for performing other signal time-base
compensations, such as correcting time-base errors
present in other time varying information signals,
eliminating differences in relative time--bases of
signals and purposely altering the time-base of signals.
With particular reference to Figure 1, the uncorrected
color television signal reproduced by the disc recorder
is applied to the input of an analog-to-digital (A/D)
converter 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.
Th:Ls representatlon signal i9 further processed to be
eventually coupled error~free to a digital-to-analog
(D/A) converter 113, which decodes the digitized signal
and reconstitutes at an output 114 the television
signal in analog 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 into the signal to
form the desired composite television signal at its
output 117.
In the compensator 110 of the invention, the
encoding A/D converter 111 provides a multi-bit word
representation of the lncoming signal at output 112
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;
each time the converter ]]1 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 the lncoming television signal, such that
a succession of binary words is developed a~ its
output 112, each word consisting of a number of binary
bits, which bits together represent a particular
amplitude level in a binary format. In general, this
~peration of analog-to-digital conversion 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 an
input coupled to line 119 and, :ln response to a
succession of reEerence clock signals received over ~ ;
lines 121 and 122, issues a reconstituted or decoded
analog television signal to an output processor 116,
which communicates the corrected television signal to
the output 117. In accordance with this invention,
the time-base error compensation is achieved by deriving
a clock signal from a time-base component included ln
the television signal so that the clock time o~ the
derived clock signal is coherent with the time-base
component. The derived clock signal is employed to
clock the A/D converter 111 to sample the uncorrected
television signal and effect the encoding of the
television signal into the digital binary word
representation. After encoding, the digitized television
signal is time buffered and decoded at the D/A converter
113 by a clock slgnal at a clock time coherent with a
reference time-base signal, such as a reference color
subcarrier. As a result of such buffering and decoding,
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the 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 information-signal-related clock signal
from the color synchronizing burst time-base component
located on the back porch of each horizontal line
blanking intervaL. The derivation is 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 burst available at output 112 of ~`
the ~/D converter 111. The store 123 provides a digital
memory for a ylurality of binary words corresponding to
the amplitude levels of the signal's color burst at
sample times. By storing the binary words available
during the sampling of the signal's color burst,
sufficient information is memorized in store 123 for
repetitively regenerating a full cycle of the color
burst such that a continuous signal identical to the
uncorrected television signa:L's color burst can be
developed lastlng beyond the duration of the signal's
color burst. The derived clock signal i9 obtained by
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 color burst
samples stored in the recyclable store 123 remains
in-phase with the color burst, hence, uncorrected
television signal, the A/D converter 111 is first
clocked during the sampling of the television signal's
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color burst and storing of the resulting samples by
a clock signal at a clock time coherent with the
reference clock signal. Thus, the A/D converter 111 -~
must be clocked by 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 lll receives over line 118 a clock control
signal locked in-phase to the reference clock signal.
The A/D converter lll is clocked by the second,
derived clock control signal received over line 118
during a following recycling mode, which lasts for the
remalnder of the horiæontal llne 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 X3
reference clock source 128. Switching device 126 is
actuable to a second or recycllng state, which connects
line 118 to the derived clock s:Lgnal provided by a
digital memory circuit 129 over line 127. In the ;~
recycling mode, switching device 126 connects the
clock input (CL) of the A/D converter 111 with a X3
signal clock 131 providing a clock output for memory
circuit 129. The X3 signal clock 131 is responsive
through a bandpass filter 132 to an output of a D/A
converter 133. The D/A converter 133 converts or
reconstitl!tes the binary word representations of the
signal color burst recycled in the recyclable store 123 ~ ~-
into an analog form. Accordingly, the signal available
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from the 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 provide a center ;;~
frequency equal to that of the color burst of the
signal being corrected, which in the case of a NTSC
standardi~ed 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 ~3
signal clock 131 has been found to provide an advantageous ;
,:
restoration oE the color burst frequency following the
various conversion and digital storage manipulations.
If a number oE 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.
2~ As indicated above, switching device 126 of
switching means 12l~ is normally in its illustrated
second or recycling state, connecting X3 si.gnal 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 television signal with the recycled
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 13~ '
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is provided for detecting at the input of the compensator
110 the occurrence of each horizontal sync pulse (SIG H)
appearing during the blanking interval of each horizontal
line of the television signal. The OUtpllt 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. After an
interval of approximately 6 microseconds, the pulse
generator 136 issues a pulse lasting about 2.0 micro- ;
seconds for actuating the switching device 126 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, separa~or 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 devlce
126 is actuated to its sampling and storing state during
the middle interval of the color burst interval. It is
desired to 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 representa-
tion 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 smal~ changes in the location of the color
burst on the back porch of the horizontal blanking interval.
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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 color burst in the incoming ~ -
television 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
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 ensulng 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/~ 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 write enable
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input (l~) of ~he random access memory 139 each ~:ime
a X3 reference clock is received from line 122. ~s
long as write enable signals are received by the
random access memory 139, the binary words issued by
the A/D converter 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 lssues a rcset command over
llne 146. The first reset command issued following r
the command provided over line 138 by the burst
detector 137 is coupled to disable the previously l~
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 ;
2~ prevented from receiving further binary word
representations of the television signal after fifteen
samples of color burst have been received. The counter
145 also serves to recycle the address generator 141.
Each time the address generator 141 issues an address
signal, the enabled counter 145 ls clocked by a X3
reference clock signal received from line 122 to ~ ;~
examine via a line I47 the address lssued by the
address generator 141 and coupled to its data (D) input.
When the counter 145 detects the issuance of the last
of fifteen address slgnals issued by the address
generator 141, it issues a reset command to the address
generator over line 146. The counter also uses this
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reset command lnternally to reset itself to again
e~amine address siKnals issued by the address
generator 141. In this manner, the address generator
141 is continuously cycled through the fifteen
addresses identifying ~he 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 ;
information signal must be sampled, the clocking or
sampling Ere~uency must be at least two times the
maximum signal frequency which the system is to pass
without substantial degradation. Furthermore, the ~ ;
clocking rate and storage capacity of the random
access memory 139 must be selected such that the number
of digiti~ed samples stored in the random access
memory 139 is equivalent to an integral number of full
cycles of the time-base component of the signal, i.e.,
e~ual 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 ull cycles of the timing component
of the signal, which when recycled results in the
generation of a continuous 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
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encoding clock signal to have a frequency three times
the color burst ~requency and by storing fifteen
samples of the color burst. Accordingly, in the
exemplary embodiment, X3 signal clock 131 includes a
frequency multiplier for multiplying by a factor of three
the continuously regenerated color burst signal ~eveloped
by store 123, D/A converter 133 and the bandpass filter
132. It is observed that the frequency of the encoding
clock signal emplo~ed during the sampling and storing
mode must be nominally equal to the established encoding
rate, although the phase may differ from the derived
clock signal in accordance with the time-base error
of ~he signal being compensated~
In the embodiment of Figllre 1, the basic reference
time-base signal i5 the reference color subcarrier
available, for example, from the studio reference source
for synchronizing all of the studio equipment for
broadcast purposes. This reference color subcarrier i9
applied to a reference signal processor 148 which is a
conventional component providing for compensation of
Eixed delays existing in cables and the like, and for
developing the necessary phase alteration of the
reference signal for European color systems, such as
- P~L (phase alternating line). The output of the
processor 148 provides the basic reference time-base
signal relative to 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 of three by a Erequency multiplier incLuded
in the X3 reference clock source or generator 128.
Since a Xl reference clock signal is required by the
mb/, - 21 -
7~
most preferred form of the compensator 1]0, a Xl
reference clock genera~or 149 is coupled to receive
the reference time-base signal from the processor 148
and provides over line 121 the required Xl reference
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 this ~ -
instance, A/D converter lll is designed to provide an
8-bit word at each clock time, with thesc n blts
provLding a 0 to 256 amplitude level capacity for the
dlgLtal representation of the incoming television slgnal.
~ecyclable digital store 123, therefore, has a 15-word
capacity, again with each word consisting of 8-bits.
As there are ~hree samp]ing 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 hori~ontal sync
pulse, the memory 139 is commanded by write clock
generator 143 (upon ~he occurrence of burst) to write
or store the binary words occurring at output 112 of
the A/D converter 111 at the instant of each X3
referenced 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
mb/~ - 22 -
7~
,
word at O-ltpUt 112. The 2 microsecond p~llse Lssued
by the pulse generator 136 temporarily sets the
switching device 126 in its sampling and storing state,
thereby coup]ing the X3 reference clock 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 ~.`
10 generator 141 following the issuance of the 2 microsecond ~ ~
pulse and issuing the reset command to the write clock . ~.;
generator 143. The reset command disables the write
clock generator, thereby removing write enable s.Lgnals .
from the random access memory 139.
Follow:ing 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 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 swltch 151 is connected by line 153 to
3b the output o~ the random access memory 139 and the
other input is connected by the by-pass line 154 to
the llne 112 at the input of the store 123. ~hile set
., ' ' ~'~
mb/~l; - 23 - :;:
to provide l~rite enable sigllals during the samp]ing
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 condition 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 stored color burst words to be coupled to
the D/A converter 133 for derLvation oE the lnformation-
signal-related clock signal~ The provision of the
by-pass switch 151 enabLes the X3 clock signal clrcuits
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 Ln the binary words stored in the memory
139 being repetitively read in such sequence throughout
the remaining duration of the horizontal line interval
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. ~igure 3A illustrates the
condition that would exist if the incoming color
television signal was without error. During the
sampling and storing interval, the X3 reference clock
causes the sampling of the signalls color burst in the
A/D converter 111 and the storing of the sample values
mh/!J - 2~1 -
7~
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, as illustrated by Figure 3B, the sample values
represented by the binary words obtained from the A/~
converter 111 will be different. This difference exists
because of the time-base difference between the reference
time-base slgnal and the Lnconling televlsion signal,
hence, the different sample points during tlle 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,
the signal clock derived from the filter output will
always be in-phase with the time base component contained
in the television signal regardless of time-base changes
or errors that may occur therein.
While in this instance a random access memory,
address generator and counter means have been employed
for recyclable store 123, it will be appreciated that
other digital storage circuitry may be used in place
thereof. For example, a recycling shift register is
capable of providing the function of store 123, as will
be recognized by those skilled in the art.
To simplify the avoidance of errors in the
re-timing of the digital representations of the television
signal output by the A/D converter 111 during the
mb/)c - 25 -
" ,, ..;;
recycling mode, a time buffer 156 is employed having a :~
l-word serial to 3-word parallel converter 157 at its
input and a complementary 3-word paralle] to l~word
~serial converter 158 at its output. The converters
157 and 15.S 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 converter 157 receives each of the succession of
binary words at a clock rate of 3 times the recycled
signal color burst by applying the clock pulses from
the X3 clock sources available on line 118 to the clock
(CL) input of th;s converter as indicated. The converter
157 i5 constructed to store three of the binary words
generated at ou~put 112 in a serial Eashion and :Ls of the
kind wherein each new word ad~led to the converter shifts ~.
the last word out leaving the converter always loaded
with three full binary words. The serially loaded
information is transferred in parallel fashion to the '
converter 158 through a clock isolator 163 (See Figure 4) ~-
included in the time buffer 156. During each line
interval of the input television signal, the transfer
time to the clock isolator 163 occurs at the clock time
determined by clock pulses developed by a lX signal
clock 159 (See Figure l). The lX signal clock is
connected to the output of bandpass filter 132 so as
to generate a clock pulse signal at the recycled color
burst rate, which is the rate of the color burst as it
occurs at the beginning of the line interval. In
particular, the lX signal clock 15~ is provided by
limiting the filter output and using a positive going
leading edge of the thereby generated square waveform
to provide the clock pulse~s. Rach positive going leading
"~
ml~ 26 -
edge of the limited regenerated color burst identifi.es
the beginning of a cycle of the color burst. The lX
signal clock 159 is connected to the time b~lffer 156
over a line 161. In this manner, the clc)ck isolator
163 receives in response to each applied clock pulse
the full contents of the converter 157, which as
discussed above carries at all times three ful.l binar~7
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 :l57 is a 2~-bit :;
word coupled to the :Lnput of the clock :isolator 1.63.
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 thereof with
reference to different incoherent clock signals, thereby
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 (WE~ 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
1~9 and coupled to a read address (~A) input of the
isolator 163 over line 121.
mb/~.) - 27 -
~, .
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. ~:
Converter 158 is the complement of converter 157 .-
in that it provides a parallel in-serial-out transfer of the
digital word information received from converter 157 through
clock isolator 163. Converter 158 thus reconverts the ;~
digital information to a l-word serial format, however,
- 10 in this instance the serial words are clocked 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. Tllese 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 foregoing manner, the digital compensator
of this invention functions to synchroniæe an incoming
in~ormation sigllal with a reference or standard tlme-base
signal. It is observed that the range of time correction
is, in the present embodiment, a period corresponding
to a full cycle of the time-base component ~Iore
particularly, in the case of a color television signal,
the correction range is one cycle of the color burst
frequency which is one divided by 3.58 megahertz or
approximately .28 microseconds. If the phase error of
the incoming television signal is likely to exceed this
range, such as may occur ~7hen reproducing television
signals from tape recorders, then the slgnal issued at
output 114 will be shifted so as to synchronize the
. ~:
mb /J ,, - 28 -
phase of the color burst component to the reference
color subcarrier. Ilo~ever, the horizont.l] sync ot the
te]evision signal will be improperly phased relative
to the reference horiæontal sync signa]. For certain
applications, such as in conjunction with disk recording
equipment, the correction range of one full cycle of
color burst, or 0.28 microseconds provided by this
embodiment, 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~er ].58, as shown Ln Figure 4. The memory 16
corrects the tlme-base oE the signal by Increments equal
integral whole numbers of the period of one cycle 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 ls enabled
at its enable input (WE) to write the 24-bit word and
the generator 166 is clocked by the lX reference clock `
on llne 121. The contents of the memory ]64 is read
according to the address provided by a read address
generator 167. The read address supplied by generator
167 is determined by the relative time of the occurrences
of the horizontal sync pulses of the incoming signal
and of the reference. The relative time of occurrences
is determlned 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 ]68 counts at the rate of
color burst. The output of the counter 168 is coupled
mb/! - 29 -
to the set (S) input of ehe read address generator 16
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
sequential addresses of the memory 164. The capacity
of the memory 164 can be adjusted as desired. For a
correction of at least one horizontal line interval,
i.e., about 63.5 microseconds, the memory 164 is
arranged to have a capacity of 256 words. Each word -~
represents a time of one period of color burst~ i.e., ;
about 0.28 microseconds. Therefore, a capacity o 256
words ~ill 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
wlll 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
compensation of information signals lacking or having
time-base components in a form other than an alternating
amplitude time-base signal. For example, a monochrome
.
~ mb/ ~ 30 -
7~
televisioll slgna]. may be corrected -in accordance wi.th
the principles of the present invention by inserting ~.
an artificial burst or pilot signa] consisting of a burst
of alternating amplitude variations into the telev:ision
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, wherein the ~
horizontal 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 F-igure 5, a modif:ication oE the
systcm of Figure 1 :Ls ill.us~rated :Eor compensatlng 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 the uncorrected monochrome horizontal sync
provided by the sync separa~or 134. An output line 173
of generator 171 is provided for issuing a burst of
al.ternating amplltude tlme-base information for insertlon
into the monochrome television signal at a summing
junction 174 by a lead 177 from a gate 176. Junction 174
is provided by a conventional signal summing 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 lll, in this instance. Such arrangement is
operable only by the absence of a color burst occurring
in the incoming signal. To thi.s 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.
~part from the f~ct that in the system of ~.
Figure 5 the burst signal i6 arti~icially generated
and inserted, this system for use with monochrome ~`
television signals functions in substantially the same
manner as 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 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 from sync separator 13~ the horizontal. sync
pulse of each monochrome televislon l:Lne as it apl)ears
ln 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
frequency of oscillation equal to that of the standard
color burst, which in turn is nominally equal to the
20 frequency of the reference color subcarrier. The phase
of the output burst signal generated by ri.nging
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 13~. The flip-flop 179
has a pair of outputs 181 and 182 corresponding to
opposite 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
30 oscillator 171 such that it develops a 180 phase change
: at each television line so as to conform the arti.f:icially
generated burst signal to the standard phase alternation
mb/) - 32 -
;~:
existing between color burst and sync timing in a NTSC
standarcl;zed color television si&nal.
Accordingly, flip-flop 179 responds to each
horizontal sync pulse by changing states. In response
to a first horizontal sync pulse 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. Phase controlled ringing
oscillator 171 is designed to respond only to output
-transitions from outputs 181 and 182 e~hibiting 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 place of the burst detector 137.
~:,
,,', 1 '.:
mb/J~J ~ 33 ~
