CA1141022A - Time base compensator - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Timing errors in a color television signal equal to a fraction of the nominal period of one cycle of color burst arc corrected by clocking an analog-to-digital converter during each horizontal line interval of the television signal with two clock signals having the same nominal frequency equal to a multiple of that of the color burst. During the color burst interval of each horizontal line, the analog-to-digital converter is clocked by a stable time base reference clock signal and the obtained digitized color burst is stored in a recyclable digital memory. Following the color burst interval, the stored digitized color burst is regenerated for the remainder of the horizontal line and a clock signal derived for use in clocking the analog-to-digital converter. The digitized television signal provided by the analog-to-digital converter is written into a clock isolator at times determined by the clock signal derived from the digitized color burst and, thereafter, read from the clock isolator at times determined by the reference clock signal. Timing errors exceeding the nominal period of one color burst cycle are corrected by writing the digitized television signal from the clock isolator into a following random access memory and incrementally adjusting the memory's read time in accordance with such errors measured in steps equal to the peri of one color burst cycle.
The stable time base reference clock signal is coupled to control the signal processing following passage through the clock isolator.

Description

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~14~ 2 FIELD OF THE INVENTION

In general, this invention relates to techniques of altering the time-base of time varying signals. More particu-larly, however, it concerns a time altering technique especially suited for electronically correcting undesirable time-base differences in time varying signals.

BACKGROUND OF THE INVENTION

During the processing of time varying electrical signals for signal transformation, analysis or correction, ( :
frequently, the time-base of the signal must be altered or compensated. For example, signal time-base compensation is commonly employed to correct~undesirable time-base differences in signals having recurrent time-base synchronizing components.
Alteration of a signal time-base to correct undesirable time-lS~ base differences is particularly important when the signalundergoes transformations between different domains, such as occur in recording an~d reproducing signals on magnetic or other forms of record media. During the recording and reproduction ~ processes, the time function of the signal is transformed into ;~ 20 a space function and then back into the time function. As the ~ signal undergoes the transformations, timing or time-base errors :~ are often intxoduced to the signal. The dynamic or time variant class of such time-base errors prevents the achievement of the necessary transient-free and time-stable signal reproduction ::
required in high resolution signal processing systems. For example, time-stable signal generation is desirable in all television signal processing systems and highly stable generation mandatory in systems used to prepare television signals for public transmission. ,~

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Two techniques are employed to correct undesirable time-base errors in signals reproduced from a record medium;
electro-mechanical and electronic. Electro-mechanical techniques are employed to correct gross time-base errors and achieve such correction by synchronizing the operation of the signal recording and reproducing equipment. Electronic techniques are employed to correct smaller residual time-base errors not corrected by the electro-mechanical devices and achieve such correction by time displacing the signal after its reproduction. It is the electronic technique of time-base error correction to which the present invention is relevant Heretofore, electronic signal time-base alteration systems have employed adjustable time delay devices inserted in the signal path to correct time-base errors. In such systems, the time-base error is measured and the amounk of time delay inserted in the signal path adjusted to compensate for and, thereby, correct the measured time-base error. One particular type of system which enjoys widespread use has a voltage variable delay line in which lumped constant inductors and voltage variahle capacitive diodes are interconnected in a delay line configuration. A voltage, corresponding to the measured time-base error~ is applied to the ~ariabl~ d~odes to fix the necessary delay for correcting the time-base error.
A description of a voltage variable delay line type signal time-base alteration system can be had by reference to U. S.
Patent No. 3,202,769.
In another type of electronic signal time-base al~eration system, a number of fixed delay lines, or a single delay line with a series of taps spaced therealong, are arranged in combination by electronic switches. Time-base errors are _"5 _ ,,,~,.. .

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corrected by operatin~ the switches in accordance with the measured error to selectively insert the necessary corrective delay in the signal path. A fixed delay line type signal ti~.e-base alteration system is described in U. S. Patent No. 3,763,317 and a tapped delay line type signal time-base alteration system is described in U. S. Patent No. 3,7~8,-36-8-.-Recently, digital delay devices, such as clockedstorage registers, have been used in systems for correcting time-base errors in analog signals. In the digital systems, the analog signal being corrected is di,gitized, corrected and reconstituted. Correction is performed hy entering or writing :
the digitized signal in an adjustable storage register at a fixed rate determined by the frequency of a reference clock signal. ~he storage register is operated to correct time-base errors by reading the signal from the register at an adjusted ~ ::
faster or slower 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. I~ ma~netic tape recorders, such ; 20 incremental time-base changes are commonly caused by anomalies their operation and most commonly when switching hetween maynetic transducer heads.
~ ~ In sign~l time-hase alteration systems, especiall~
;~ those arranged to eliminate time-base errors and provide a high degree of signal time-base stability, it has been the practice to cascade coarse time-base correction device~ and fine time-base correction devices. Voltage variable delay line systems have been used to provide the desired fine time-base correction while switched delay line systems have been used to ~rovide the coarser time-base corrections. Because all such delav line systems are analog devices, they are prone to drift and have other disadvantages characteristic of analog devices. Incre-mental time-base changes that occur as a result of anomalies in the operation of tape recorders often cause errors or costly interruptions in the performance of signal processing operations because of the inability of these time-base error correction devices to respond to the incremental changes. Also, if a large range OI time-base errors is required to be corrected, large and complex correction systéms are necessary.
Considerable advantage is therefore to be gained by utilizing a technique to perform signal time-base compensation that is able to af~e~t all time-base alterations, including incremental, without error. Additional advantages will be realized in the performance of such signal time-base compensa-lS tion by first altering the signal time-base by any fraction of a known increment required to bring the signal within an intesra1 number of known increments of the desired time-base reference ~ and, thereafter, alterlng~the signal time-base by such integral ; numher of known increment to adjust the signal to the desired time base.
' SUMM~RY OF THE I~JENTION

A feature of this invention ls the utilization of digital techniques to alter signal time-base which enable digital circuits to be employed ~hat are far less expensive ; ~ 25 to construct and maintain than analog circuits. Another feature of this invention is that time-base compensation can be per-formed withQut the need of an analog measurement of the amount of compensation desired, thereby avoiding all of the disadvan-tages characteristic o~ analog measurement circuitry. It is .,, " ,, - ~4~ 2Z

yet another feature of this invention to re-time the signal by a fraction of a known increment by temporarily storing the signal in a time buffer at a time adiusted in accGrdance wi.h the desired time-base change while maintaining the storage retrieval time fixed relative to an established time-base reference. A f~ ther eature of this invention is that further .:
incremental alterations in the time-base of a signal can be - performed without error by adjusting the further storage retrieval time of the signal in accordance with a desired time-base change while maintaining the storage entry time fixed relative to an established time-base reference. Still another feature o~ this invention i5 that alterations in the time-~ase of a signaI grester than one principal division of the time-base, as determined by the period of one cycle of the signal's time-base component, can be performed by first altering the signal time-base by any desired amount correspond-ing to a fraction of the princ}pal time-base division and thereafter further incrementally altering the signal time-~ase by any desired~amount corxes~ondlDg to an integral number of principal time-base div~ SiOIIS.~ Yet another feature of this invention is that time-base alterations are performed hy the use of a derived con rol signal which reduces the effect of noise to a large degree. These and other features of this invention provide particular advantages when the invention is ernployed to eliminate time-base errors in television signals reproduced ~rom video recording equipment.
; In accordance with this invention, an information signal whose time-base is to be altered, i.e., compensated, is - sam~led to obtain representations oE the si~nal. The informa-tion signal must contain or be provlded with a time-base ,- ",,j component, appearing at least at intervals of the information signal. A timing or time-base reference, such as a clock signal having a frequency that remains stable relative to the nominal frequency of the time-base component associated with the uncompensated information signal, is initially employed to control the`sampling time and rate. The reference clock signal must be generated relative to the occurrence of the information signal so that at least a portion of the information signal's time-base component is sampled a number of times. Such sampling must be sufficient to permit regeneration of the time-base component from the representations thereof.
As the time-base component is sampled under the control of the stable reference clock signal, the represen-tative samples are stored and, thereafter, used to regenerate ; a representation of the time-base component, which is frequency stable relative to and phase coherent with the original time-base component associated with the uncompen-sated information signal. An information clock signal is derived from the regenerated time-base component so that its frequenc~ and phase characteristics are stable relative to those of the regenerated, hence, original time-base component associated~ with the information signal. During the interval of the information signal following the portion of the time-base component from which the information clock signal is derived, the derived information clock signal is used to time or control additional processing of the information signal for the introduction of the desired amount of time-base alteration.
The use of a derived clock signal obtained in the above described manner provides particular advantages in the further processing of an information signal, such as, for example, a television signal, to alteriits time-base for the purpose X

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OI eliminating timing differences or time-base errors that commonly occur ir, such signals. ~en employing the techni~ue of this in~ention to eliminate time-base errors, that occur in the television signal, the frequency and phase of the reference clock signal is maintained fixed and the derived clock signal is employed to time the fur-ther sampling of the informaiion si~nal during the interval following the portion of the infor-mation signal's time-base component rrom -~hich the information clock signal is derived. To eliminate time-base errors from color television signals, the information clock signal is derived from a re~enexation of the color synchronizing burst that occurs at the beginning of each horizontal line in-terval of the composi~e color television signal. The thusly derived clock signal is employed to time the sampling of the video information signal component following the synchronizing interval located at the beginning of each horizontal line of the tele-vision signal.
~ Following the further sampling, the obtained repre-- sentations of the video si~nal a e written in a clock isolator ;20 or time buffer at times determined by the derived clock signal.
Thereafter, the video signal representations are read ~rom the buffer at a time deterrnined by the fi~T~frequency and phase 5 `!~
reference clock signal. Tn this fashion, the time buffer serves to re-time the video signal representations relative to the reference clock signal. The oriyinal form of the video signal may be reconsti-tu~e~ from the re-timed sampled representa-tions read from the buffer.
'rhe use of a clock signal derived from a reyeneration of the time-base component of an information signal to time the further processing or sampling of the inormation signal is one C~

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of the fund~mental features of this invention that facilitates the alteration of signal time-base. As described hereinabove, the derivation of the information clock signal in this manner assures that the frequency and phase of the derived clock signal will always be precisely related to those of the time-base component contained in the information signal. Therefore, the time-base of the derived clock signal will follow changes in - the time-base relationship of the information signal and timing reference. Because the tlme-bas~ of the derived clock signal is precisely locked to that of the information signal and the derived clock signal is used to control the further sampling of the information signal, the information signal will always be further sampled at the same points during its interval regardless of the time-base relationship of the information signal and timing reference. Changes in the time-base ; relationship of the information signal and timing reference - will not change the sample points during the information signal interval. This enables the thusly sampled information signal to be re-timed relative to any desired time-base refer-e~ce, regardless of changes in the time-base relationship of the information signal and timing reference. As will become readily apparent upon consideration of the following detailed descriptions of preferred embodiments of the signal time-base alteration technique of this invention, the derivation and use of the information clock signal to further sample the informa-tion signal enables outstanding advantages to be realiæed in the implementation of the technique, the most significant of which is the precise time-base error corrections of television signals with a high degree of reliability.

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zz Ordinarily, the time-base component of an in crmation signal is a simple periodic signal. However, some information signals, such as television signals, have several time-base ~ O ~ S
components arranged to define principal~and sub-periods of the information signal and intra-period time-base conditions thereof.
Because such time-base components have different frequencies, it is possible in some circumstances for sub-periods to appear properly aligned relative to a reference even though the higher ordered periods are not properly aligned. To avoid the possible harmful effects that could be caused by a false indication of proper time-base alignment, the highest frequency time-base :
component is selected for deriving the information clock signal.
Signal time-base compensation up to one cycle of the highest frequency time-base component is automatically prov1ded by the 15; above described technique of using the derived information clock signa] to further sample the information siynal. If signal time-base compensations greater than one cycle of the highest frequency time-base component are necessary to achieve the proper time-base alignmer1t, the information s1gnal is further examined to dekermine the numher of full cycles it must further be altered to properly align its time-base. Tne required further alteration is accomplished by storing the s~mpled representations in a memory for a number of cycles correspond-ing to -th~ determination. Preferably, the further alteration is performed after the sampled representations have passed through the time buffer.
In addition to altering the time-base of an information signal to eliminate undesirable time-base differences, the signal time-base compensation in accordance ~Jith this invention can be employed to introduce wanted time-base chan~es in an information 2~

signal. Such wanted time-base changes are introduced by al~er-ing the time-base of the reference clock signal in accordance with the want~d time-base changes. In other respects, the signal time-base compensation of this invention is performed as described above with reference to the elimination of time-base errors. ~ltering the time-base of reference cloc~ signal causes a change ln the time-base relationship of the reference clock signal and time-hase component contained in the informa-tion signal. As previously explained, such relative ; 10 time-base change introduces a comparahle time-base difference between~the time-base of the sampling of the information signal :: :
~ and that of the time-base altered reference clock signal.
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Therefore, reading the samples of the information signal from the time buffer~ at times determined by the time-base altered reference clock signal results in the re-timing of the infor-:
~ mation signal relative to the altered reference signal and, :: - : , thereby, the introduction of the wanted time-base changes in the information slgnal.
As will be appreciated~from the foregoing, signal 20~ time-base compensation in accordance with the present invention is easily adaptable to digitalization and, therefore, is able to benefit from the advantages that can be gained by the use of digital circuits. Furthermore, the ability to alter the time-base of an information signal first by a fractiQn of a kno~n time increment or principal time-basa division and, thereafter - by any amount equal to an integral number of such increments, régardless of the size of the time-base alteration, has the advantage of avoiding the limitations associated with ~ascading analog time-base alteration devices.

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BRIEF DESCR-PTION F T~.'P, DRAWIN~S

The ~oregoing as well as other featurGs and aavan-tages of the signal time-base alteration technique of this invention will become more apparent upon the consideration of the following detailed description and claims toyether ~,~7ith the accompan~ing drawings o~ which:
Figure 1 is a block diagram of a digital time-base ::
compensator in accordance with this invention adapted for a color television signal;
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Figure 2 is a detailed block diagram illustrating the construction of the recyclable digital store of the compensator of Figure 1;

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Fiqures 3A and 3B are timing diagrams illustrating the operation of the signal time-base~compensation in accordance with this invention in elimlnating time-base errors from color television si.gnals;~
Figure 4 illustrates circuits in block form that permit the time-base compensator of Figure 1 to correct errors :; greater than one cycle of the signal's color synchronizing : 2G burst.

Figure 5 illustrates circuits in block form that permit the time-base compensator embodiments of Figures 1 and 4 to operate when the incoming signal is a monochrome television signal.

s~ 3 z DESCRIPTION OF PREFERRED EMBODIMENTS
The signal time-base compensator 110 ~ accordance with the present invention is shown in Figure 1 as arranged to eliminate time-base errors present in a color television information signal reproduced by a video recorder (not shown), such as a magnetic disc recorder. However, it will be appreciated that the principles of this invention are equally applicable for performing other signal time-base compensations, such as correcti~g time-base errors present in other time varying information signals, eIiminating differences in reIative time-bases of signals and purposely altering the time-base of signals. With particular reference to Figure 1, the uncorrected color television signal repro-duced ~y the disc recorder is applied to the input of an analog-to-digital,,(A/D), c~nverter 111,, which is operable to provide at its output 112 an encoded signal in the form of a pulse code modulated representation of the television signal. This representation signal is further processed to be eventually coupled error-free to a digital-to-analog (DjA) converter 113, which'decodes the digitized signal and reconstitutes at an output 114 the television signal in analoy form. Because the synchronizing components included in the television signal issued by the D/A converter 113 usually are misshaped and contain undesirable transients as a result of their passage through the compensator 110, the television signal is coupled to an output processor 116 of the type commonly used in video recorders. Such processors 116 operate to strip the synchronizing components from the incoming television signal and insert new properly shaped and timed synchronizing components in~o the signal to form the desired composite television signal at its output 117.

2;2 In the compensator 110 of the invention, the encoding A/D converter 111 provides a multi-bit word representation of the incoming signal at output 112 each time the converter 111 is clocked by a clocking signal applied over a line 118, as shown. The converter 111 is clocked to sample the instantaneous analog amplitude of ~he l.ncoming televi.sion signal, such that a succession of binary words is developed at its output 112, each word consisting of a number of binary bits, which bits together represent a particular amplitude level in a binary : 10 format. In general, this operation of analog-to-digital con-version may be referred to as pulse code modulation of the incoming signal. The reverse of this operation is performed by the decoding D/A converter 113. The decoding converter 113 receives the binary encoded words at ~n input coupled to line llg IS and, in response to a succession of reference clock signals received over lines ]21 and 122, issues a reconstituted or decoded analog television signal to an output processor 116, which co~nunicates the corrected televlsion signal to the output 117. In accor-dance with this invention, the time-base error compensation is 20~ achieved by deriving a clock signal from a time-base component included in the ielevision signal so that the clock time of the derived clock signal is coherent with the time-base component. The derived clock ~ignal is employed to clock the A/D converter 111 to sample the uncorrected television signal and effect the encoding of the television signal lnto the digi~al b.inary ~ord representations. After encoding, the d.igitized television signal is time buffered and decoded ~t tne ~/A co~verter 113 by a clock signal at a clock time coherent with a reference time-base si.gnal, such as a reference color subcarrier. As a result of such buffering and decoding, the ~S

decoded television signal is rendered in-phase with the reference color subcarrier.
In the case of a color television signal, precise time-base corrections can be achieved by deriving the infor-mation-signal-related clock signal from the color synchronizing burs-time-base component located on the back porch of each horizontal line blanking interval. The derivat:ion i.s achieved by coupling to ~:~ the input of a recyclable digital store 123 binary word representations of one or more cycles of the signal's color ; 10 burst available at output 112 of the A/D converter 111. The store 123 provides a digital memory for a plurality of binary words corresponding to the amplitude levels of the signal's color burst at sample times~ By storing the binary words available durlng the sampling of the signal's 15. color burst, suf~icient information is memorized in s~ore 123 for repetitively regenerating a full cycle of the color burst such that a continuous signal identical to the uncorrected television si~nal's color burst can be developed lasting beyond the durati~on o~ ~the signal's color burst. The derived clock signal is obtai.ned b~ further processing the continuously regenerated color burst signal and is employed to digitize the remainder of the horizontal line of the television signal from which~it is regenerated.
To insure that the continuous signal, hence, derived clock signal regenerated from the c~lor burst samples stored in the rec~clable store 123 remains in-phase with the color burst, hence, uncorrected tele~ision signal, the A/~ converter 111 i5 first clooked during ~he sampling of the television . . .

;~ signal's cu'or burst and storing of the resulting samples by a - 30 clock signal a~ a clock time coherent with the reference clock ~ 'L ~

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`~ signal. Thus, the A/D converter 111 must be clocked b~ two clock control signals over line 118. The initial clocking occurs during a sampling and storing mode, preferably, lasting for several cycles of the color burst time-base component. During this initial mode, the clock input (CL) of A/D converter 111 receives over line 118 a clock control signal locked in-phase to the reference clock signal. The A/D converter 111 is clocked by the second, derived clock control signal re eived over line 118 during a following recycling mode, which lasts for the remainder of the hori-zontal line interval after the initial clocking. For these two modes of operation, a switching means generally indicated at 124 is provided having a switching device 126 disposed in a first or sampling and storing state connecting the line 118 to the clock output line 122 from a ~3 reference clock source 128. Switching device I26 is actuable to a second or recycling state, which connects line 118 to the derived clock signal provided by a digital memory circuit 129 over line 127. In the recycling mode, switchingdevice 126 con-nects the clock input (CL) of the A/D converter 111 with aX3 signal clock 13I providing a clock output for memory circuit 129. The X3 signal clock 13I is responsive through a bandpass filter 132 to an output of a D/A converter 133.
The D/A converter 133 converts or reconstitutes the binary word representations of the signal color burst recycled in the recyclable store 123 into an analog form. Accordingly, the signal available from thé D/A converter 133 appears as a continuous unfiltered replica of the input signal time-base component, which, in this preferred embodiment, is a sinusoidal color burst of a television signal. The bandpass filter 132 is set to provlde a center frequency equal to that of the color burst of the signal being corrected, which .r~
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in the case of NTSC standardized color television signal is a frequency of 3.58 megahertz. Filter 132 in its location between the output of D/A converter 133 and an input to X3 signal clock 131 has been found to provide an advantageous restoration of the color burst frequency following the various conversion and digital storage manipulations. If a number of signal color burst cycles are sampled and stored in store 123 for regenerating the derived clock signal, the filter 132 will average any noise contained in the recycled signal color burst over the number of stored cycles, thereby improving the timing accuracy of the derived clock signal.
As indicated above, switching device 126 of -switching means 124 is normally in its illustrated second ;or recycllng state, connecting X3 signal clock 131 to the clock input (CL) of the A/D converter 111 so as to control the sampling and time the encoding of the uncorrected teIevision signal with the rec~cled color burst samples derived from the signal. To provide for the actuation of switching device 126 to its other first or sampling and storing state, switching means 124 includes circuitry for detecting the occurrence of the color burst time-base component in the television signal and responsively operating device 126 in accordance therewith~ In particular, a sync separator 134 ~s provided for detecting at the input of the compensa~or 11~ the occurrence of each horizontal sync pulse (SIG H) appearlng during the blanking interval of each horizontal line of the television signal. The output of -the separator is coupled to the input of a switch control pulse generator 136. Upon the detection of the leading edge of the horizontal sync pulse, the separator 134 issues a command to the pulse generator 136.

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After an interval o~ approximately 6 microseconds, the pulse generator 136 issues a pulse lasting about 2.0 microseconds for actuating the switching device 12~ to its sampling and storing state. Thus, in response to the appearance of a horizontal sync pulse at the input to the A/D converter 111, separator 134 and pulse generator 136 cause switching device 126 to apply the encoding X3 reference clock signal to the clock input (CL) of the converter 111, which responsively digitizes a selected number of cycles of the signal's color burst. The timing of the operations of the separator 134 and pulse generator 136, as specified herein, is arranged for NTSC television signals so that the switching device 126 is actuated to its sampling and storing state during the mi~ddle interval of the color burst interval. It is dasired to ~15 arrange the sampling and storing of digital representations of the signal's color burst to occur in the middle of the color burst interval because this interval is the most accurate and reliable in representation of the color synchronizing burst frequency. In addition, the derivation of the information-signal-related clock signal is less susceptible to errors that may be introduced by small changes in the location of the color burst on the back porch of the horizontal blanking interval.
To condition the recyclable store 123 to store five cycles of the color burst digital representations, a burst detector 137 is connected to the input of the compensator 110.
Upon the occurrence of the aolor burst in the incoming televi-sion signal, the burst detector 137 issues command on line 138, which extends to the write enable input (WE) of the recyclable digital store. This command causes the store 123 to write the multi-bit binary words appearing at output 112 from the A/D

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,, ~q converter 111. The actual writing or storage operation occurs at each reference clock time determined by a clock signal input to storage 123 from X3 reference clock 128.
The ensuing operation of recyclable store 123 may be best described with reference to both Figures 1 and 2.
With reference to Figure 2, store 123, includes a random access memory 139 having conventional write and address control inputs, as indicated by (W) and (A) symbols respectively.
~ A binary word input is connected for receiving the multi-bit binary word at output 112 of the A/D converter 111. A binary word output is provided for issuing the recycled digital ~signals to line 140. An address generator 141 is responsive to a source of X3 reference clocking signals over line 122 and provides over a connection 142 address signals for write and read access to memory 139 in accordance with the generated address signal. Included within store 123 is a write clock generator 143 responsive to the command received over line 138 from burst detector 137. The command sets the write clock generator 143 to issue over line 144 write enable signals to the writ enable input (W) of the random access memory 139 each time a X3 refe~ence clock is received from line 122. As long as write enable signals are received by the random access memory 139, the binary words issued by the A/D convertar 111 will be written for storage in the memory 139. The store 123 also includes a counter 145 responsive to the command received at its reset (R) input coupled to line 138 from burst detector 137. The command resets the counter 145 for counting addresses issued by the address generator 141. The counter 145 is also reset by an internally generated command as will be described below. Each time the counter 145 is reset, it issues a reset ~.

command oYer line 146. The first reset command issued following the command provided over line 138 by the burst detector 137 is coupled to disable the previously enabled write clock generator 143 by resetting it until the next command is issued by the burst detector 137. In this manner, the random access memory 139 is prevented from receiving further binary word representations o the television signal after fifteen samples of color burst have been received.
The counter 145 also serves to recycle the address generator ld 141. Each time the address generator 141 issues an address signal, the enabled counter 145 is clocked by a X3 reference clock signal received from line 122 to examine via a line 147 the address issued by the address generator 141 and coupled to its data (D) input. When the counter 145 detects ~he issuance 1 of the last of fifteen address signals issued by the aadress generator 141, it issues a reset command to the address generator over line 146. The counter also uses this reset command internally to reset itself to again examine address signals issued by the address generator 141. In this manner, the address generator 141 is continuously cycled through the fifteen addresses identifying the locations in the random access memory 139 in which the fifteen multi-bit binary words representing the five sampled cycles of signal color burst are stored.
A further explanation of the operation of the recyclable store 123 will be provided herein with a description of an actual operating sequence of the compensator 110.
In selecting the rate at which the incoming informa-tion signal must be sampled, the clocking or sampling requency must be at least two times the maximum signal ~requency which the system is to pass without substantial degradation.

_~_ Furthermore, the clocking rate and storage capacity of the rando~.
access memor~ 139 must be selected such that the number of digitized samples stored in the random access memory 139 is equivalent to an integral number of full cy~les of the time-base component of the S signal, iOe., equal to the product of the number of samples per cycle or period of the time-base component and an integral number of the cycles. With the clocking rate and storage capacity thusly selected, the random access memory 139.carries an integral number of digital representations of full cycles of the timing component of ~he signal, which when recycled results in the generation of a con-: tinuous clock signal during the recycling mode. In the case of a color television signal, both:the storage capacity and the sampling ~: rate criteria are advantageously satisfied by selecting the encoding clock signal to have a frequency three times the color burst fre-quency:and by storing fifteen samples of the. color burst. Accord-ingly, i.n the exemplary embodiment, X3 signal clock 131 includes a frequency mu~ltiplier for multlplylng by~a~factor of three the con-tinuously regenerated color burst signal developed by s~-ore 123, ~ D/A converter 133 and th~ bandpass fil~er 132. It is observed tha~
:: 20: the frequen~y o the encod.ing clock signal employed during the . sampling and storing mode must he nominally equal to the established encoain~ rate, although the phase~may differ from the dPrived clock signal in accordance with the time-base error of the signal .~eing :~
: ~ compensated.
In the embodiment of ~igure 1, the basic reference time-base signal is the referance color subcarriar available~
for example, from the studio reference source for synchronizing ~ all of the studio equipment for broadcast purposes. This : reference color subc~rr.ier is applied to a refer~nce signal processor 148 which is a conventional component providing for compensation o fixed delays existing in cables and the like, --~3.--~, "~
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and for developing the necessary phase alteration of the reference signal for European color systems, such as PAL
~phase alternating line). The output of the processor 148 provides the basic reference time-base signal relative to S which the compensator 110 operates to compensate the incoming television signal. Because of the need of a X3 reference clock signal, the frequency of the basic reference time-base signal is multiplied by a factor o three by a frequency multiplier included in the X3 re~erence clock source or generator 128.

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~ 10 Since a Xl reference clock signal is required by the most , ~ preferred form of the compensator 110, a Xl reference clock :
generator 149 is coupled to receive the reference time-base signal from the processor 148 and provides over line 121 the required Xl re~erence clock signal.
In accordance with the foregoing selection of encoding and decoding clock rates, the A/D converter 111 functions to develop a separate binary word at each of the three clock times occurring during the period equal to one cycle of the color burst. In ~his instance, A/D converter 111 is designed to provide an 8-bit word at each clock time, with these 8 bits providing a 0 to 256 amplitude level capacity for the digital representation of the incoming television signal.
Recyclable digital store 123, therefore, has a 15-word capacity, again with each word consisting of 8-bits. As there are three sampling points for each cycle of the color burst, the random access memory 139 of the store 123 provides for storing five full cycles of the digitally represented color burst. In operation, while the pulse generator 136 issues the 2 microsecond pulse in response to the detection of the horizontal sync pulse, the memory 139 is commanded by write clock generator 143 (upon .~ ;;-, ~3 , ~, the occurrence of burst) to write or store thé binary words occurring at output 112 of thé A/D converter 111 at thé -instant of each X3're~erenced clock signal received over line 122. With reference to Figure 2, this operation in particular provides for address generator 141 accessing a new word store in memory 139 in response to each of the X3 reference clock pulses, each newly accessed word store receiving the instantaneous bit conditions of the binary word at output 112. The 2 microsecond pulse'issued by the pulse generator 136 temporarily sets the switching device 126 in its sampling and storing state, thereby coupling the X3 reference cloc~ signal to clock the A/D converter ;~ 111.
After the five cycles of the digitized color burst have been stored the storing operation is terminated by the counter 145 detecting via line 147 the fifteenth address generated by the address generator 141 following the issuance of the 2 microsecond pulse and issuing the reset command to the write'cIock generator 143. The reset command disables the write clock generator, théreby removing write enable signals from the random access memory 139.
Following the termination of the sampling and storing mode, the'address generator 141 continues to access memory 139 in response'to the X3 reference clocking signal over line 122, repeating in sequence'the same fifteen word locations accessed during the write operation. This causes the stored 8-bit words to be successively read out over output line 140 to the D/A converter 133. The memory 139 is permanently disposed in an active read mode, such that the stored binary words are:'continuously read out over line 140. The read function is operational during the storage of new digital information received from ...

the A/D converter 111 by the operation of a by-pass switch 151. The switch 151 has two inputs and one output. One input of the by-pass switch 151 is connected by line 153 to the output of the random access memory 139 and the other input is connected by the by~pass line 154 to the line 112 at the input of the store 123. While set to provide write enable signals during the sampling and storing mode, the write clock generator 143 conditions the by-pass switch 151 to connect lines 112 and 140, thereby, passing directly to the output the binary words being stored in the memory 139. At the end of the sampling and storing mode, the write clock generator 143 is disabled, hence, placing the switch 151 in a co,ndition to couple output - line 153 of the memory 139 to the line 140. The switch 151 remains in this condition during the entire recycling mode, enabling the s~ored color burst words to be coupled to the D/A converter 133 for derivation of the information-signal-related clock signal. The provision of the by-pass switch 151 enables tha X3 clock signal circuits to be readied for the generation of derived X3 clock signal.
During the recycling mode, the address generator 141 and counter 145 function together to cause the repetitive generation of the same address sequence. This results in the binary words stored in the memory 139 being rep~titively read in such sequence throughout the remaining duration of the horizontal line i~terval following the color burst.
Figures 3A and 3B illustrate the manner in which the derived clock signal is generated to be in-phase with the time-base component of the information signal from which it is derived. Figure 3A illustrates the condition that would exist if the incoming color television signal was without error.

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.. . .

41~;~2 . .
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, .. ~ "
During the sampling and storing interval, the X3 reference clock causes the sampling of the signal's color burst in the A/D converter 111 and the storing of the sample values in the recyclable store 123. Because the incoming television signal is without error, the first sample of each cycle of the signal's color burst occurs at the beginning of the color burst cycle.
Upon the recycling of the fifteen words stored in store 123, the output of the filter 132 will be in-phase with the color burst contained in the incoming television signal If a time-base error exists in the incoming television signal, asillustrated by Figure 3B, the sample values represented by the binary words ob~.ained from the ~/D converter 111 will be : ::
different~ This difference exists because of the time-base difference between the reference time-base signal and the incoming television signal, hence, the differ~nt sample , points during the color burst cycle. ~pon recycling the fifteen words stored in store 123, the regenerated color burst signal output by the filter 132 will be in-phase with the color burst contained in the incoming television signal :~ :
Hence, tne signal clock derived from the filter output will alway~ be in-phase with the time-base component contained in the television si.gnal regardless of time-base changes or errors that may occur therein.
~hile in this instance a random access memory, ~ 25 address generator and counter means have been employed for ; ~ ~ recyclable .store 123, it will be appreciated that other digital storage circuitry may be used ln place thereof.
For exa~ple, a recycling shift register is capable of providing the function of store 123, as will be recognized by those

3~ skilled in the art.

,~' zz To simplify the avoidance of errors in the re-timing of the digital repr~sentations of the television signal output by the A,/D CGnVerter 111 during the recycling mode, a time buffer 156 is employed having a l-word serial to 3-~ord parallel converter 157 at its input and a complementary 3-word parallel to l-word serial converter 158 at its output. The converters 157 and 158 are shown in Figure 4. The succession of individual binary words developed at output 112 are passed into the serial-in-parallel-out converter 157. This conver-ter 157 receives each of the succession of binary . ~ ~
words at a clock rate of ~ times;the recycled signal color burst by applying ~he clock pulses from the X3 clock sources available on line 118 to the clock (Chj input of this converter as indicated.
The converter 157 is constructed to store three of the binary words generated at output. 112 ln~a serial fashion and is of the kind 15~ wherein each new word added to the~converter shifts the last word out leaving the converter always loaded with three full binary words.
The~serlally loaded;informatlon~is~transferred ln parallel fashion :: ~: : : .
to the converter 158 through a clock isolator 163 (See Figure 4) inc~luded in the time buffer 156. During each line interval o the 20~ înput television~signal, the transfer time to the clock isolator 153 occurs at the clock time de~ermined by clock pulses developed hy a lX signal clock 159 ~S~ee Figure l). The lX signal clock is connected to the output of bandpass filter 132 so as to generate a cloc~ pulse signal at the recycled color burst rate, which is the rate of the ~25~ color burst as it occurs at the heg-nning of the line interval.
In particular, the lX signal clock 159 is provided by limiting the filter output and using a positive going leading edge of the thereby gen~.ated square waveform to provide the clock pulses.
Each positive goiny leadincl edge of the limited regenerated color burst identifies the beginning of ~ cycle of Z

the color burst. The lX signal clock 159 is connected to the time buffer 156 over a line 161. In this manner, the clock isolator 163 receives in response to each applied clock pulse the full contents of the conYerter 157, which as discussed above carries at all times three full binary words generated by the A/D converter 111 at output 112. Moreover, the three words received in a parallel format by the clock isolator 163 correspond to the three words developed during one cycle of the regenerated color burst.
The output of the converter 157 is a 24-bit word coupled to the input of the clock isolator 163. The isolator is able to simultaneously read and write the 24-bit words.
Because the isolator 163 is able to read and write simultaneously, the clocking operations can occur on the input and output sides lS thereof with reference to different ~ = clock signals, ~hereby providing time buffering and the ability to re-time signals. To write or store the output of the converter 157, clock signals generated by signal clock 159 are coupled by line 161 to write address (WA) and write enable IWE) inputs of the isolator 163. This clock signal is in-phase with the color burst of the uncorrected television signal. The stored 24-bit words associated with each cycle of the time-base component are read or output from the isolator 163 in response to lX reference clock signals provided by a reference clock generator 149 and coupled to a read address (RA) input of the isolator 163 over line 121.
By clocking the isolator 163 with the two clock signals, the phase of output of the isolator will be re-timed and synchronized to the reference color subcarrier phase.

_~ _ Converter 158 is the complement of converter 157 in that it provides a parallel-in-serial-out transfer of the digital word infor~ation received from converter 157 through clock isolator 163. Converter 158 thus reconverts the dlgital 5 information to a l-word serial format, however, in this instance the serial words are clockecl out of the converter 158 at a clock time determined by the lX reference clock applied to converter 158 over line 121, as indicated in Figure 4.
These serial words are applied over line 119 to the input of the D/A converter 113 and, thereupon, decoded under the control of the 3X reference clock present on line 122. The D/A
converter 113 reconstitutes the desired analog signal at output 114 synchronized to the reference subcarrier phase.
In the foregoincl manner, the digital compensator of this invention functions to synchronize an qncoming information ; signal with a reference or standard~time-base signal. It is observed that the range of time correction is, in the present embodiment, a period~correspon~ing to ~ full cycle of the time-base component. More particularly, in the case of a color :~ :
television signal, the correction range is one cycle of the colcr burst~frequency which~is one divided by 3.58 megahertz ; or approximately .28 microseconds. If the phase error of the incoming ~ si~gnal is likely to exceed this range, such as may occur when reproducing television signals from tape recorders, then the signal issued at output 114 will be shifted 50 ~S to synchronize the phase of the color burst component to the reference color subcarrier. ~Iowever, the horizontal sync of the television signal will be improperly phased relative to the reference horizontal sync signal. For certain applica-tions, such as in conjunction with disk recording equipment, ~r~, `''~.' .
.

.
the correction range o one full cycle of color burst, or0.28 microseconds providea by this embbdiment, is adequate without the aid of additional time-base error compensating systems.
If larger time-base errors are likely to be present, a random access memory 164 is inserted between the clock isolator 163 and the parallel-to-serial word conver-ter 158, as shown in Figure 4. The memory 164 corrects the time-base of the signal by increments equal integral whole numbers of the period of one cyale of color burst. This is accomplished by writing the 24-bit word at addresses in the memory 164 determined by a write'address generator 166.
The memory 164 is enabled a~ its enable input (WE) to write the 24-bit word and the generator 166 is clocked by the lX
reference~clock on line 121. The' contents of the memory 164 is read according to the address provided by a read address generator 167. The read address supplied by genera-~; tor 167 is determined by the relative time of the occurrences of the hbrizontal sync pulses of the incoming signal and of the refer'ence.'~The'relative time'ofoccurrences is determined by a counter serving as a horizontal sync comparator 168. The counter 168 is started to count in response to the reference horizontal sync and is stopped by the occurrence of the television signal's horizontal sync. The counter 168 counts at the rate of color burst. The output of the counter 168 is coupled to the set (S) input of the read address generator 167 and changes by setting the output read address in accordance with the number in the counter 168 following the occurrence of the television signal's horizontal sync.
The successive 24-bit words are written at sequen-tial addresses of the' memory 164. The capacity of the memory 164 .....

can be adjusted as desired. For a correction of at least one horizontal line inter~al, i.e., about 53.5 microseconds, the memory 164 is arranged to have a capacity of 256 words. Each word reprasents a time of one period of color burst, i.e., about 0.28 microseconds. Therefore, a capacity of 256 words will provide in excess of 63.5 microseconds of storage. The read address generator 167 is set relative to write address generator 166 so that if the signal horizontal sync and reference horizontal sync are in phase~ identical addresses generated by the two generators will be separated in time equivalent to that required to cycled about one-half the capacity of the memory, with the write address generation in advance of the read address generation. For a one horizontal line interval correction capacity, the separation is about 32 microseconds.
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The foregoing construction and operation of this invention applies to a system for correcting an information signal having a recurrent time-base synchronizing component in the form of a burst of alternating amplitude variations, such as color burst. ~This invention is also capable of time-base error compensa~ion of information signals lacking or having time-base components in a form other than an alternating amplitude time-base signal. For example, a monochrome tele-vision signal may be corrected in accordance with the principles of the present invention by inserting an artificial burst or pilot signal consisting of a burst of alternating amplitude variations into the television signal during a blanking interval thereof. In particular, such a burst signal may be added to the back porch of each blanking interval accompanying a horizontal line of the monochrome television signal, wherain --~32--"~

the harizontal sync pulse serves as the time-base component to which the inserted pilot signal is selected to have a predetermined phase relationship.
With reference to Figure 5, a modification of the system of Figure 1 is illustrated for compensating a monochrome television signal by -inserting an artificial burst signal consisting of a burst of alternating amplitude time-base ~ .
information. Burst insertion is provided by a ringing oscillator burst generator 171 having an input controlled by 10~ the uncorrected monoahrome horizontal sync provided by the sync separator 134. An output line 173 of generator 171 is ~; provided for issuing a burst of alternating amplitude time-base information for insertion into the monochrome television ; signal at a summing junction 174 by a lead 177 from a gate lS 176. Junction 174 is provided by a conventional signal sum-ming circuit. By this arrangement the generated artificial :
burst signal is inserted in the monochrome television signal prior to application of the incoming signal to the encoding A/D converter 111, in this instance. Such arrangement is - 20 operable only by the absence o a color burst occurring in the incoming signal. To this end, a connection is made from the output of the burst detector 137 to gate 176 to disable the gate whenever a color burst is detected in the incoming signal.
Apart from the fact that in the system of Figure 5 the burst signal is artificially generated and inserted, this system for use with monochrome television signals functions in substantially the same manner as ~ ~n~-r~n~-~e~Son~~th the system of Figure 1 used for color television signals.
The artificial burst generator 171 is designed so as to generate a burst signal having the same frequency and phase relationship 3~

as a color burst, so that the standard reference color subcarrier may be employed as the reference time-base signal in the monochrome circuit of Figure 5. This is achieved in accordance with the present invention by generator 171 receiving fxom sync separator 134 the horizontal sync pulse of each monochrome television line as it appears in the incoming television signal and~employing the leading edge of the ;~ horizontal sync pulse to trigger a phase controlled ringing circuit designed to provide a f~equency of oscillation e~ual to that of the standard color burstj which in turn is nominally equal to the frequency of the reference color subcarrier.
The phase of the output burst signal generated by ringing generator 171 is controlled in accordance with the output of a divide by 2 flip-flop 179 having an input responsive to the leading edge of the horizontal sync pulse as developed by sync separator 134. The flip-flop 179 has a pair of outputs - 181 and 182 corresponding to oppoæite sides of flip-flop 179, ; thus issuing signals which are 180 opposed. The purpose of divide by 2 flip-flop 179 is to drive phase controlled ringing 20 ~ oscillator 171 such that it develops a 180 phase change at each television line so as to conform the artificially generated burst signal to the standard phase alternation existing between color burst and sync timing in a NTSC standardized color television signal.
Accordingly, flip-flop 179 responds to each horizontal sync pulse by changing states. In response to a first horizontal sync~received from separator 134, output 181 will switch from a low to a high state while output 182 will simultaneously switch from a high to a low state. The next horizontal sync pulse will cause an opposite transition.

~!! 3 ~

~L4~1~Z2 Phase controlled ringing oscillator 171 is designed to respond only to output transitions from outputs 181 and 182 exhibiting a low to high change in state.
As each artificial burst appears at output 173 following the horizontal sync pulse, the 2 microsecond pulse output provided by the pulse generator 136 actuates the gate 176 by disposing it in its set condition. Also a mono/
color switch 183 is set to couple the pulse from the pulse ~ .
generator 136 to control the recyclable store 123 in pIace of the burst detector 137.

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Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of processing video information signals to reduce time base errors, said method comprising the steps of:
a. generating a first clock signal train having a variable rate determined by the frequency of first predetermined portions of said video signals;
b. generating a second clock signal train having a variable rate determined by the frequency of second predetermined portions of said video signals, said second predetermined portions having a frequency range substantially less than the frequency range of said first predetermined portions;
c. sampling said video signals at intervals defined by said first clock signal train;
d. temporarily storing said sampled signals at said intervals defined by said first clock signal train; and e. fetching said stored signals at intervals defined by said second clock signal train.

2. The method of claim 1 wherein said step (c) of sampling includes the step of converting said video signals to digital form, and further including the step of (f) reconverting said fetched signals to analog form.

3. A method of processing video information signals to reduce time base errors, said method comprising the steps of:
(a) generating a first clock signal train having a variable frequency determined by the frequency of first predetermined portions of said video signals;
(b) generating a second clock signal train having a variable frequency determined by the frequency of second predetermined portions of said video signals, said second predetermined portions having a nominal frequency value sub-stantially less than the nominal frequency value of said first predetermined portions;
(c) sampling said video signals at intervals defined by said first clock signal train;
(d) temporarily storing said sampled signals at said intervals defined by said first clock signal train; and (e) fetching said stored signals at intervals defined by said second clock signal train.

4. The method of claim 3 wherein said step (c) of sampling includes the step of converting said video signals to digital form, and further including the step of (f) reconverting said fetched signals to analog form.