Note: Descriptions are shown in the official language in which they were submitted.
Zl~i
FIELD OF THE INVENTION
In genera], this invention relates to techniques
of altering the time-base of time varying signals. More
particularly, ho~ever, 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. Alteration 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 si~nal 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 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 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 of system which enjoys widespread use has a
voltage variable delay line in which lumped constant inductors
and voltage variable capacitive diodes are interconnected in a
delay line configuration. A voltage, corresponding to the
measured time-base error, is 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
si~nai time-base alteration system can be had by reference to
U.S. Patent No. 3,202,769 issued August 24, 1965 to Columbia
Broadcasting System, Inc.
In another type of electronic signal time-base
alteration 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 corrected by operating the switches in accordance
with the measured error to selectively insert the necessary
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corrective delay in the signal path. A fixed delay line type signal
time-base alteration system is described in U.S. Patent No. 3,763,317
issued Cctoker 2, 1973 to Ampex Corporation and a tapped delay line type
signal time-base alteration system is descriked in U.S. Patent No.
3,743,366 issued July 24, 1973 to Ampex Corporation.
Recently, digital delay devices, such as clocked storage
registers, have been used in systems for correcting time-base errors in
analog signals. In the digital systems, the analog signal being corrected
is digitized, corrected and reconstituted. Correction is performed by
entering or writing the digitized signal in an adjustable storage register
at a fixed rate determined by the frequency of a reference clock signal.
The storage register is operated to correct time-base errors by reading
the signal from the register at an adjusted faster or slcwer rate, depending
upon the time-base error. This technique of constant write rate and
variable read rate cannot handle large discontinuous or incremental time-
base changes in the signal, In magnetic tape recorders, such incremental
time-base changes are commonly caused by ancmalies in their operation and
most com~only when switching between magnetic transducer heads.
In signal time-base alteration systems, especially those
arranged to eliminate time-base errors and provide a high degree of signal
time-base stability, it has been the practice to cascade ooarse time-kase
correction devices 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 provide
the coarser time-kase corrections. Because all such delay line systems
are analog devices, they are prone to drift and have other disadvantages
characteristic of analog devices.
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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 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 of
time-base errors is required to be corrected, large
and complex correction systems are necessary.
Considerable advantage is therefore to be gained
by utilizing a technique to perform signal time-base
compensation that is able to effect all time-base
alterations, including incremental, without error.
Additional advantages will be realized in the performance
of such signal time-base compensation by first altering
the signal time-base by any fraction of a known increment
required to bring the signal within an integral number
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 utilization
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 regenerating a time-base
component of an information signal, comprising; means
for sampling an interval of the time-base component at
times during the interval thereof determined by a
time-base reference signal; means responsive to the
time-base reference signal for receiving and storing
the samples of the time-base component; and means for
regenerating the stored samples of the time-base
component in the order of their storage at times
determined by the time-base reference signal during the
time between the successive sampled intervals of the
time-base component.
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 re8enerate
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 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 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 is employed to time the further
sampling of the information signal during the interval
following the portion of the information signal's
time-base component from which the information clock
signal 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 interval 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 reference
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 further 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 hereinabove, the derivation
of the infor~ation 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 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 tlme-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 detailed
descriptions of preferred embodiments of the signal
.
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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 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.
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
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 for
a number of cycles corresponding to the 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 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.
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ll~gZ16
As will be appreciated from the foregoing,
signa] 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 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 alteration
devices. -
:
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BRIEF DESCRIPTrON OF TIIE DRAWINGS
__ __ _ . ___
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 detailed 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 of Figure l;
Figures 3A and 3B are timing diagrams
illustrating the operation of the signal time-base
compensation in accordance with this invention in
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 synchronizing 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.
DESCRIPTION OF PREFERRED EMBODIMENTS
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. Uowever, it will be appreciated
that the principles of 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 l, 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.
This representation signal is 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 incoming signal at output 112
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~42~l6
each time the converter 111 is clocked by a clocking
signal applied over a line 118, as shown. The converter
lll is clocked to sample the instantaneous analog
amplitude of the incoming television 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 format. In general, this
operation 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, in response to a
succession of reference 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 cloc~ signal from a time-base component included in
the television signal so that the clock time of 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 signal 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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2l~;
the decoded television signal is rendered in-phase
with the reference color subcarr-ier.
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 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
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 signal's color burst can be
developed lasting beyond the duration of the signal's
color burst. The derived clock signal is 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 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 received over line 118
during a following recycling mode, which lasts for the
remainder of the horizontal 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 X3
reference clock source 128. Switching device 126 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, 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 me~ory
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
reconstitutes 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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1~?4~
from the D/~ converter 133 appears as a continuous
unfiltered replica of the input signal time-base
component, which, in this preferred embodiment, is a
sinusoidal co]or 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
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 recycling 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 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 134
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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 outl~ut 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, 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 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 small 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 AtD
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 write enable
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input (W) of thc random access memory ]39 each time
a X3 reference 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 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 issues a reset command over
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 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 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 the issuance of the last
of fifteen address signals 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 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
information signal must be sampled, the clocking or
sampling frequency 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 digitized 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.,
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 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 frequency 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 developed
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 the signal being compensated.
In the embodiment of Figure 1, the basic reference
time-base signal is 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 is
applied to a reference signal processor 148 which is a
conventional component providing for compensation of
fixed delays existing in cables and the like, 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 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 frequency multiplier included
in the X3 reference clock source or generator 128.
Since a Xl reference clock signal is required by the
.
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most preferred ~orm of the compensator ]10, 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 requlred 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 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 the 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 -
.
2~
worcl at output 1]2. The 2 microsecond pulse is.sued
by the pulse generator 136 temporarily sets the
switching device ]26 in its sampling and storing state,
thereby coupling the ~3 reference clock signal to
clock the A/D converter ]11.
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 clock
generator 143. The reset command disables the write
clock generator, thereby 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 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
mb/ - 23 -
'
. . .
to provide write enable signa]s during the sampling
and storing mode, the write clock generator 143
conditions the by-pass switch 15] 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 derivation of the information-
signal-related clock signal. The provision of the
by~pass switch 151 enables the 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 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. Figure 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 signal's color burst in the
A/D converter 111 and the storing of the sample values
mb/ - 24 -
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 co]or 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/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 different sample points during the color
burst cycle. Upon 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/,~ - 25 -
11~4~
recycling mode, a time bufEer 156 is emp]oyed having a
l-word serial to 3-word paralle] converter 157 at its
input and a complementary 3-word parallel to l-word
seria] 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 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 this converter as indicated. The converter
157 is constructed to store three of the binary words
generated at output 112 in a serial fashion and is of the
kind wherein each new word added 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 1). 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 159 is provided by
limiting the filter output and using a positive going
leading edge of the thereby generated square waveform
to provide the clock pulses. Each positive going leading
mb/,j - 26 -
?s2~
edge oF the limite~ regenerated color burst identifies
the beginning of a cvcle of the color burst. The lX
signal clock 159 is connected to the time buffer 156
over a line 161. In this manner, the c]ock 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 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 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 si~nals provided by a reference clock generator
149 and coupled to a read address (RA) input of the
isolator 163 over line 121.
mb/ , - 27 -
11~4~1~
By c]ock;ng the isolator ]63 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 ]58 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,
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. 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 foregoing manner, the digital compensator
of this invention functions to synchronize an incoming
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 corresponding
to a 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 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 when reproducing television
signals from tape recorders, then the signal issued at
output 114 will be shifted so as to synchronize the
mb/~,, - 28 -
2~6
phase of the color burst component to the reference
color subcarrier. However, the horizontal sync of the
television signal will be improperly phased relative
to the reference horizontal sync signal. 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
converter 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 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 is enabled
- at 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 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 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
mb/)- - 29 -
- : : . . .. . . . : .,,:.. .
- : . .
:. : .: ,. - : . ' - . -. . .:
:~ .' , ,, . . . : , : .: . , : ' :
2~
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
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 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
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
.. . . - ~ .
.
~1(J 4~1~
television signa], may be corrected in accordance with
the princlplcs 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, 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 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 1 ' "
ringing oscillator burst generator 171 having an input -
controlled by the uncorrected monochrome 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 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 111, in this instance. Such arrangement is
operab]e only by the absence of 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.
mb/, - 31 -
, '' ' . ' . ~ . . :
. ' :
11~4~1~
Apart from the fact that .in the system of
Figure 5 the b~lrst signal is art:ificially generated
and inserted, thi.s system for use with ~onochromc
televisi.on signals functions in substantial.].y 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 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
frequency of oscillation equal to that of the standard
color burst, 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
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
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
mb/' - 32 -
- - : - . . ~, . :
::
'
Zl~
existing betweell color burst and sync timing in a NTSC
standardi~ed coLor televi.sion si&nal.
Accordingly, flip-flop 179 responds to each
horizontal sync pu].se by changing states. In response
to a first horiæontal. 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 hi.gh .
to a lo~ 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 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 place of the burst detector 137.
: ,
mb/J 3
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