TIME BASE CORRECTION OF TELEVISION SIGNALS

Abstract: In a time base correction system, a composite color television analog signal derived from a prerecorded tape is converted by an analog-to-digital converter (14) to a digital signal having horizontal sync and color burst signal components. The digitized sync and burst signals are processed on a line-by-line basis to provide an error signal between input video and a sampling clock in the form of a binary control word. The error signal is applied to a digital frequency synthesizer (22) which generates a clock at the subcarrier frequency to achieve precise adjustment of frequency and phase of the A/D sampling clock with respect to the video signal.

Description: Technical Field This invention relates to a time base correc­ tion system and is particular to a system that employs digital circuitry for measuring and correcting frequen­ cy and phase errors of a recorded color television signal during playback. Background Art In color television recording systems that employ NTSC signals including horizontal sync and color burst components, inter alia, it is usually necessary to correct the timing errors caused by variations in tape speed, for example, or for frequency and phase differences appearing in the signal that is recorded. In order to provide the desired correction, prior art video systems generally employ a phase locked loop (PLL) including a voltage controlled oscillator (VCO) to measure the difference in phase and frequency between the VCO clock frequency and the burst signal component of the composite color television signal. Also to eliminate time base errors and provide a high degree of signal stability, it has been proposed to use digital circuits to digitize the video signal and to generate an error signal that is stored in memory to be used for frequency and phase correction. In such systems, correction of the phase error is accomplished in the time base correction network at the output of the system on a line-by-line basis. However, the phase error is cumulative and builds up along each horizontal line, so that it becomes more difficult to compensate for large phase errors. In addition, the phase locked loops used in prior art systems do not operate suffi­ ciently fast to follow the rapid changes in the frequency and phase of a video signal recorded on, for example, a fast moving tape. SUMMARY OF THE INVENTION The invention overcomes the above disadvan­ tages while providing various added advantages over present schemes for correcting frequency and phase errors. Thus, the invention provides an improved time base correction system employing digital circuitry in which adjustments for frequency and phase errors are minimized. Further, the invention provides a time base correction system that may follow and correct frequency and phase errors relatively rapidly to avoid buildup of large errors. In providing the above features, a sampling clock is used which has a precise and continuous relationship to the color subcarrier of the incoming video signal. More particularly, the invention time base correction system provides phase correction of a color video signal which includes a color burst component by employing a clock signal preferably having a frequency that is a multiple of the subcarrier frequency. The clock signal is locked in phase to the color burst component of the video signal and phase correction is effectuated with reference to a digital sampled burst signal so that any errors in the analog portion of the video signal are servoed to zero in a closed loop arrangement. In a specific embodiment of the invention, a time base correction system converts components of an off-tape analog video signal to digital pulses. The digitized sync and burst components of the video signal are filtered and processed in a high speed digital signal processor to generate a frequency control word. A digital frequency synthesizer generates a clock signal at subcarrier frequency in response to the frequency control word, and the clock signal is applied to an analog-to-digital (A/D) converter which operates at four times the subcarrier frequency. The output of the A/D converter is a series of binary words at the multiplied frequency rate representing samples of successive portions of the video signal. The samples are fed back to the input of a closed feedback loop constituting sync and burst filters, a processor, synthesizer and phase locked loop. The clocked binary data samples of off-tape video signal are passed to a time base correction memory for further time base correction of the sampled video signal. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described with refer­ ence to the drawing in which: FIGURE 1 is a block diagram of the clock generator of this invention; FIGURES 2a-c are waveforms representing the timing relation of early and late video signals; FIGURE 3 is a sinusoidal waveform represent­ ing the input analog burst signal and the digitized filtered samples A and B; FIGURE 4 is a schematic block diagram of the high speed digital signal processor of FIG. 1; FIGURE 5 is a schematic block diagram of the frequency synthesizer of FIG. 1; FIGURES 6a-c depict burst samples represent­ ing change with reference to clock phase error; FIGURES 7a and 7b represent the phase error angle ϑ between clock samples and burst zero crossings; FIGURES 8a-c show the outputs of the phase accumulator, digital-to-analog converter and low pass filter respectively, of the frequency synthesizer of FIG. 5; and FIGURE 9 portrays the relation of the 4x subcarrier sync samples relative to a defined slice level obtained from digitized sync edge detection. DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to FIG. 1, a composite analog video signal is derived from a prerecorded magnetic tape and passed through a delay circuit 10 providing a delay of one horizontal line, i.e., 63.5 µsec. plus 3.5 µsec, or a total delay of approximately 67.0 µsec. This delay produces a "late video" signal and allows a look ahead at the phase of the burst signal at the beginning and at the end of each horizontal line to determine whether any changes in phase have occurred during the period of the horizontal line under consid­ eration. A delay of more than one television line enables the video input to switch dynamically between the delayed burst signal and the burst of the undelayed signal, so that both burst signals are applied through a gated switch 12 to an analog-to-digital converter 14. The delayed analog signal is applied to the gated switch 12 which is normally in the "late video" posi­ tion, but switches to "input video" or "early video" during the input video burst time. The output signal from the timed switch contains active video and burst components and the leading edge of the horizontal sync from the late video signal, as well as the burst component of the input or early video signal. The timing relationship of the early and late video signals and the combined signal obtained from the output circuit of the gated switch 12 are delineated in FIG. 2. The combined signal is passed to the ana­ log-to-digital converter 14, which samples the video output signal from the switch 12 at four times the subcarrier frequency, digitized to nine-bit words. Each digital word having sync and burst is processed by the time base correction circuit of this invention to determine and compensate phase and frequency errors that occur between the input video and a 4f sc (4x subcarrier frequency) clock. The digitized signal, containing the early burst and late burst components, is passed through a sync low pass filter 16 and a burst bandpass filter 18. The filter 18 averages about six cycles of each burst to reject chroma noise. For each burst, the bandpass filter 18 produces two filtered samples, designated as "A" and "B", which are 90° apart, as illustrated in FIG. 3. As shown in FIG. 3, the filtered samples A and B comprise the odd and even samples respectively, which are averaged about a baseline by adding the differences between each pair of samples in one complete cycle. The filtered burst samples A and B are applied to a high speed digital signal processor 20 which is illustrated in detail and will be described with reference to FIG. 4. The signal processor 20 calculates the sync and burst phase errors relative to the clock generated in a closed feedback loop, which includes the processor 20, a digital frequency synthesizer 22 that operates under control of a fixed reference clock or crystal oscillator 24, and an analog phase lock loop 26 operating at a frequency 4 times the subcarrier frequency f sc . The feedback clock signal is returned via lead 25 to the converter 14 and to the filters 16 and 18, and allows precise adjustment of frequency and phase of the digitizing clock. The burst samples A and B are used to develop a signal that represents the phase error between the signal obtained from the phase lock loop 26, which provides a four times subcarrier sampling clock (4f sc ) and the zero crossings of the off-tape burst signal. The burst samples are changed with relation to the clock phase error as depicted in FIG. 6. Sample A is negative when the clock is advanced in phase, and positive when the clock is delayed in phase. When there is no clock phase error, burst sample A is at zero, and burst sample B is displaced by 90°, as shown in FIG. 6c. Since sample A does not follow the phase error in a linear fashion, a clock to burst phase calculator 32, shown in FIG. 4, calculates the phase error angle ϑ between the 4f sc clock samples and the burst zero crossings, employing the equation recited in FIG. 7. The error signal ϑ represents the phase error between the clock and burst over 360°, and is indepen­ dent of burst amplitude. It is noted that ϑ depends only on the ratio of sample A to sample B. The arc­ tangent of the ratio of these two values define the sampling position of the burst. The arctangent of is valid only for ϑ over the range of ±90°. To obtain phase error over the entire 360° range, the calculator 32 senses the sign of sample B and adds 180° to ϑ if sample B is negative. The clock phase error value for the late burst is directed to a switch 64 which enables line-by-line phase correction of the clock by the frequency synthesizer. The clock-to-burst calculator 32 passes the phase error values from early burst to a burst phase subtractor including a register and subtractor circuit 36. The subtractor circuit gives the phase error across a line. The clock phase error across a line is then applied to a constant scaler 38, and an 40 which forms a frequency accumulator with register 42 that is timed by the horizontal clock pulse. The K scaler 38 converts the phase error across a line to frequency error. If there is no frequency error between the video signal and the subcarrier clock signal, then there is no accumulated phase error across a horizontal line. Velocity compensation is achieved by setting the clock subcarrier frequency to be equal to the offtape video subcarrier frequency at the start of each line The clock determines the frequency error between the input video and the clock by measuring the phase error that accumulates across a line relative to the current subcarrier clock. The phase error is obtained by comparing two consecutive bursts of the input video to the current clock. If no frequency error exists, the clock-to-burst phase measurement of one burst is the same as for the next burst. The burst phase subtractor 34, 36 serves to calculate the phase error across a line by subtracting the clock-to-burst phase values of the two consecutive bursts. The clock frequency error signal from the K scaler 38 represents the amount by which the frequency of the clock must be changed to match the input signal frequency. The current clock frequency value is main­ tained by the frequency accumulator, consisting of the adder 40 and shift register 42. For each horizontal line, the frequency error is added to the current subcarrier clock frequency to form a new clock fre­ quency, which replaces the previous frequency value as the register 42 is clocked for each line. The fre­ quency accumulator allows the clock to lock its fre­ quency to that of the input signal in a closed loop configuration. If the clock is low in frequency, the frequency error will be positive and the current clock frequency will increase until the error is zero. Similarly, if the clock is high in frequency, the error will be negative and the clock frequency will decrease until the error is zero. The output of the frequency accumulator is a 20-bit frequency control word that is passed to the frequency synthesizer 22. The frequency synthesizer 22, which is the digital equivalent of a voltage controlled oscillator (VCO), operates to correct the phase of the 4x subcar­ rier clock on a line-by-line basis. As depicted in FIG. 5, the synthesizer includes a phase accumulator 43, consisting of an adder 44 and register 46, under control of the reference clock 24, which provides a fixed 20.46 MHz timing signal. The updated frequency control word from register 42 of the processor 20 is fed to the phase accumulator 43 of the frequency synthesizer 22. FIG. 8a illustrates the phase accumu­ lator operating at the subcarrier frequency of 3.58 MHz. If using another format than the NTSC standard, for example the PAL or PAL-M formats, the phase accumulator is programmed for 4.43 MHz by choosing a larger phase increment. The phase accumulator generates a digital ramp (FIG. 8b) at the desired output frequency (f sc ). The phase accumulator, which is clocked at a constant rate by the crystal oscillator 24, provides an output signal to register 46 of a 20-bit binary number repre­ senting 360° of phase. Each time the phase accumulator is clocked by the 20.46 MHz clock, the phase advances by an amount equal to the value of the frequency control word. The phase continues to increase until it exceeds 360°. At this point, the accumulator overflows and the phase wraps around to a number less than 360°, derived from the current phase and the phase increment represented by the control word. For example, if the current phase is 350° and the phase increment or control word is 27°, the accumulator overflows to 17°. The phase will cycle through 360° of range at a rate proportional to the phase increment or frequency control word. The frequency of the phase accumulator output signal is directly related to the value of the phase increment or the frequency control word. The digital ramp waveform generated by the phase accumulator is converted to a digital representa­ tion of a sine wave form by the sine lookup PROM 48. The PROM converts the digital signal from the phase accumu­ lator to a sine wave amplitude, and functions to convert 360° of phase to one full sine wave cycle. The output signal from the sine PROM is a digital word that is converted by the digital-to-analog converter 50 to a series of voltage levels which represent the instanta­ neous amplitude of the sine wave, as depicted in FIG. 8b. The output from the digital-to-analog converter is sinusoidal, but contains sampling steps at the 20.46 MHz clock rate, which are removed by a low pass filter 52 with a 7 MHz cutoff frequency. The result is a pure fundamental sine wave having the correct frequency of 3.58 MHz, such as shown in FIG. 8c. Since the video signal is digitized at 4x the subcarrier, (4f sc ), the sinusoidal subcarrier frequency from the tape must be multiplied by 4. This is accom­ plished in the phase lock loop 26, which conventionally contains a phase comparator, a loop filter, a voltage controlled oscillator (VCO), and a divide by 4 counter. The phase lock loop 26 settles rapidly to the line-by-­ line phase corrections that are applied to the tape subcarrier phase via switch 64 and thus has a very high bandwidth. Also, as a result, phase noise generated by the analog VCO is suppressed. The digital sync low pass filter 16 operates to average several adjacent video samples to form each output samplee, and is used in conjunction with burst to provide filtered chroma line type signals that are used by a memory control to maintain a correct color at the output of the timing base correction system. The digitized sync leading edge from late video which is applied to the filter 16 is utilized by the tape clock to generate horizontal timing signals. To obtain chroma line signals for use by the memory control of the system, the digitized late video signal that is processed by the analog-to-digital converter 14 is applied to the digital sync low pass filter 16. The filtered signal is compared in a comparator 56 which generates coarse sliced sync signals that go high when the video sync samples cross below a predetermined slice level. As shown in FIG. 9, the detected digitized sync leading edge signals are sampled at the 4f sc rate and accurate timing informa­ tion is obtained by the clock-to-sync phase calculator 54. To find a point where the sync crosses the 50% slice level to an accuracy of a few degress of subcar­ rier frequency, the phase between the sync slice crossing and the first clock sample below the slice level, designated as L, is computed by determining the phase ϑ cs which is equal to 90° times (S-L)/(U-L), where S is the 50% slice level, U is the sample just above the slice level and L is the sample just below the slice level. The late video sync signal that is provided by the phase calulator 54 contains sync-to-burst phase information that is applied to an adder 56 which produces a measured sync-subcarrier phase (scH) of the offtape signal for each horizontal line. The measure­ ment is defined over 360° and is nominally 0° for video lines with positive burst polarity and 180° for nega­ tive burst lines. By comparing the measured sync­ subcarrier phase signal to a phase reference in compa­ rator 60, a burst polarity signal H/2 (where H is at horizontal rate) is obtained for further utilization. The phase reference is set midway between the two scH values for positive and negative burst types to afford maximum immunity to noise. In addition, a horizontal write pulse is produced by means of a comparator 58 coupled to receive the filtered sync signal from the low pass filter 16. The comparator 58 provides a coarse sliced sync signal to a burst crossing selector 62, which produces an H write pulse timed to the burst zero crossing phase. The burst zero crossing is selected by generating a one cycle wide window timed to the 50% slice point of the leading edge of the late sync signals. This window is delayed from sync so that its center is coincident with burst zero crossings of a standard scH phase signal. The positive zero crossing closest to the center of the window is selected for timing if H/2 is low, i.e., a positive burst is present, while the negative zero crossing nearest the window center is selected if H/2 is high, when a negative burst is present. There has been decribed herein a time base correction circuit for use in a television signal video recording and playback system wherein a 4x subcarrier frequency digital clock is generated and locked in phase and frequency to the input video and burst signals derived off the recording tape. The clock follows the timing variations of the offtape signal over all speed ranges of the playback machine. The clock provides a horizontal timing pulse that is timed to a selected burst zero crossing of the offtape signal. The timing pulse is used by the timing correc­ tion circuit to define the start of each horizontal line stored in a memory. All timing error measurement is based on digitized sync and burst signals obtained from the analog-to-digital converter at the input to the correction circuitry, obviating the need for a separate analog phase comparator with associated circuitry.

Claims: 1. A television signal processing system wherein a color video signal includes subcarrier frequency, sync and color burst components, characterised by means (14) for digitizing said sync and color burst signal components in accordance with a sampling clock, means (32) for measuring the difference between the phase of the color burst component and the phase o the sampling clock; and means (22-26) responsive to the phase difference to remove, in closed loop, phase errors from said digitized signal. 2. A television signal processing system as in claim 1, wherein said means (14) for digitizing said sync and color burst signals is an analog-to-digital converter. 3. A television signal processing system as in claim 1, including a high speed digital signal processor (20) for generating a frequency control word which is the resultant of the difference in phase between the color burst signal component and the sampling clock; and a frequency synthesizer (22) coupled to said processor for generating a sampling clock which has been phase corrected in response to said frequency control word. 4. A television signal processing system according to claim 3, wherein said high speed digital signal processor comprises a frequency accumulator (40,42) for generating said frequency control word, which is corrected on a line-by-line basis for application to said frequency synthesizer. 5. A television recording and playback system wherein input signals are processed, said signals including a nominal subcarrier frequency signal and color burst and horizontal sync components, the system including a time base correction circuit chracterized by: means (10) for delaying said input video signals to provide a delayed late video signal and an undelayed early video signal; a converter (14) for sampling said video signals in accordance with a sampling clock at a multiple frequency of said subcarrier and for converting said delayed and undelayed analog video signals to digital signals; a high speed digital signal processor (20) coupled to said converter for correcting the phase of the sampling clock and for providing a frequency control word; a frequency synthesizer (22) coupled to said processor for correcting the frequency of said sampling clock in response to said frequency control word; and means (26) operating at a multiple of the subcarrier frequency for locking the sampling clock to the video signal with correct phase and frequency on a line-by-line basis. 6. A system according to claim 5, including a video switch (12) coupled between said delaying means and said converter for switching between said delayed and undelayed signals at predetermined times. 7. A system according to claim 5, including a bandpass filter (18) coupled in a first channel to said converter for filtering noise from said color burst component and for producing two filtered video samples that are 90° apart. 8. A system according to claim 7, wherein said digital signal processor comprises a burst phase subtractor (36) coupled to said bandpass filter and a frequency accumulator (40,42) for correcting the sampling phase of said digitized burst signal. 9. A system according to claim 8, wherein said frequency accumulator (40,42) generates a frequency control word that is corrected on a line-by-line basis for application to said frequency synthesizer. 10. A system according to claim 8 or claim 9, including a clock-to-burst phase calculator (32) for processing filtered burst samples to develop an error signal representing phase error between the sampling clock and zero crossings of said burst signal components. 11. A system according to any of claims 8 to 10, wherein said burst phase subtractor comprises a storage register (34) controlled by a horizontal sync pulse clock, and a subtractor (36) coupled to said register and operating over a 360° range for providing a clock phase error across a horizontal line. 12. A system according to any of claims 8 to 11, further including a constant scaler (38) connected to said subtractor to provide a clock frequency error. 13. A system according to any of claims 8 to 12, wherein said frequency accumulator comprises an adder (40) and a register (42) timed by a horizontal sync clock, said adder and register being connected in a closed loop for generating a frequency control word. 14. A system according to any of claims 5 to 13, wherein said frequency synthesizer comprises a phase accumulator (43) controlled by a fixed reference oscillator (24) to produce a binary number representing up to a complete cycle of phase change and changing at a rate proportional to said frequency control word. 15. A system according to claim 14, wherein said frequency synthesizer includes a memory (48) for generating a sine waveform substantially without harmonics and having a phase representing the phase of said phase accumulator. 16. A system according to claim 15, including a digital-to-analog converter (50) coupled to said memory (48), and a low pass filter (52) coupled to said digital-to-analog converter and having a cutoff frequency so that a pure fundamental sine wave is produced. 17. A system according to any of claims 5 to 16, including a low pass filter (16) connected in a second channel to said analog-to-digital converter for filtering noise from said horizontal sync component. 18. A system according to claim 16, including means (62) for selecting burst zero crossings that are coincident with the horizontal sync pulses for generating a horizontal write pulse whereby phase adjustment may be achieved for the horizontal picture position. 19. A method of processing a television signal which includes subcarrier frequency, sync and color burst components comprising the steps of: digitizing said sync and color burst signals; generating a sampling clock that is locked in phase to said digitized burst signal; measuring the difference between the phase of the color burst component and the phase of the sampling clock to provide a correction signal; and servomechanically removing the phase error from said digitized signal in response to said correction signal. 20. A method according to claim 19, wherein the sampling clock operates at a frequency that is a multiple of said subcarrier frequency.

Inventors: WAGNER, STEVEN D.
Applicants: AMPEX CORPORATION
Application year: 1986
Main classification: H04N9
IPC codes: H04N5, H04N9