CA1104216A - Time-base compensator - Google Patents
Time-base compensatorInfo
- Publication number
- CA1104216A CA1104216A CA358,118A CA358118A CA1104216A CA 1104216 A CA1104216 A CA 1104216A CA 358118 A CA358118 A CA 358118A CA 1104216 A CA1104216 A CA 1104216A
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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 are 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 period 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.
Timing errors in a color television signal equal to a fraction of the nominal period of one cycle of color burst are 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 period 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
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 mb/;~ - 1 -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 sd/p '~ -2-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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1~ 16 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.
mb/~ _ 5 _ 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 mb/~ - 6 -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 mb/~-`'` - 8 -11~34~
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.
mb/ 9 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. -:
mbj'~-``' - 10 -- llV~Z~6 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 mb/ - 11 -' - : :. ' ' :
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 mb/ ~-` - 12 -~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, mb/ ~ - 13 --:
, 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 mb/~ 14 -21i 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 mb/`i ~ - 15 -. . ~ . . : :
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 mb/ `~ - - 16 -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.
mbl ~ - 17 -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 mbl `-~` - 18 -~ t3~2~
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 mb/~ - 19 -, 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 mb/~ - 20 -Z~
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 .
mb/ - 21 -21~
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 -.
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 mb/;~ - 1 -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 sd/p '~ -2-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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1~ 16 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.
mb/ ~ `; ~ 4 ~
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.
mb/~ _ 5 _ 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 mb/~ - 6 -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 .
mb/~ - 7 -1~4~
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 mb/~-`'` - 8 -11~34~
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.
mb/ 9 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. -:
mbj'~-``' - 10 -- llV~Z~6 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 mb/ - 11 -' - : :. ' ' :
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 mb/ ~-` - 12 -~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, mb/ ~ - 13 --:
, 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 mb/~ 14 -21i 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 mb/`i ~ - 15 -. . ~ . . : :
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 mb/ `~ - - 16 -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.
mbl ~ - 17 -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 mbl `-~` - 18 -~ t3~2~
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 mb/~ - 19 -, 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 mb/~ - 20 -Z~
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 .
mb/ - 21 -21~
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 ,
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 ,
Claims (5)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus for regenerating a time-base component of an information signal, comprising; means for sampling an interval of said time-base component at times during said interval thereof determined by a time-base reference signal; means respon-sive to said time-base reference signal for receiving and stor-ing the samples of said time-base component; and means for re-generating the stored samples of said time-base component in the order of their storage at times determined by said time-base reference signal during the time between the successive sampled intervals of the time-base component.
2. The apparatus according to claim 1 wherein the sam-pling means samples and digitally quantizes the samples of the time-base component to provide digital samples of the time-base component, and the storing means includes a recyclable digital memory and further comprising a digital to analog converter coupled to receive the digital time-base component samples pro-vided by the recyclable digital memory and provide a correspond-ing analog form thereof; and a bandpass filter means coupled to receive the analog form of the time-base component provided by the digital to analog converter and provide a filtered re-presentation thereof, the bandpass filter set with a center frequency substantially equal to the nominal frequency of the time-base component and with a bandwidth to effect averaging of noise present in the regenerated time-base component.
3. The apparatus according to claim 2 further comprising a switching means responsive during the interval that the digital time-base component samples are received by the recyclable digital memory to couple the digital time-base component samples to the digital to analog converter as said samples are received by the recyclable digital memory.
4. The apparatus according to claim 1 wherein the in-formation signal is a color television signal having time-base components including periodically occurring line pulses defining line intervals of information and a color synchronizing signal following the occurrence of each line pulse; the sampling means samples an interval of the color synchronizing signal time-base component; the storing means receives and stores the samples of the color synchronizing signal time-base component;
and the regenerating means regenerates the stored color syn-chronizing signal samples during the time between the color synchronizing signals following the occurrences of successive line pulses.
and the regenerating means regenerates the stored color syn-chronizing signal samples during the time between the color synchronizing signals following the occurrences of successive line pulses.
5. The apparatus according to claim 4 wherein the time-base reference signal has a stable time-base.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA358,118A CA1104216A (en) | 1974-04-25 | 1980-08-12 | Time-base compensator |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US46426974A | 1974-04-25 | 1974-04-25 | |
| US464,269 | 1974-04-25 | ||
| CA358,118A CA1104216A (en) | 1974-04-25 | 1980-08-12 | Time-base compensator |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1104216A true CA1104216A (en) | 1981-06-30 |
Family
ID=25669129
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA358,118A Expired CA1104216A (en) | 1974-04-25 | 1980-08-12 | Time-base compensator |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1104216A (en) |
-
1980
- 1980-08-12 CA CA358,118A patent/CA1104216A/en not_active Expired
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| MKEX | Expiry |