METHOD AND SYSTEM FOR PROCESSING DIGITAL VIDEO SIGNAL INCORPORATING PHASE-CORRECTION FEATURE

Abstract: A method and system for converting an analog video in­ put signal (V IN ) into a digital video signal (V O ) corrects for jit­ ter and other spurious input frequency errors without the need for a time base correction circuit. The phase of the vi­ deo input (V IN ) relative to a reference signal (SC) is measur­ ed at two points of time during video signal processing and the two measured phase values are compared to derive a phase error signal (S₅) indicative of the spurious phase er­ ror for a corresponding horizontal scan line. The phase er­ ror signal (S₅) is then used to adjust the phase of sampling clock pulses (CK) used to sample to analog input values of the corresponding scan line for conversion to digital values. Each clock pulse is phase-shifted only slightly, as the de­ rived spurious phase error is distributed evenly among all of the clock pulses for the corresponding scan line. Prefer­ ably, the phase error is distributed linearly across the corresponding scan line and mirror-symmetrically about the center of the scan line.

Description: BACKGROUND OF THE INVENTION The present invention relates generally to a method and a system for processing a video signal. More particularly, the invention relates to a method and system for processing input video signal, which has sampling points of uniform phase so as to be vertically aligned on a screen. When an analog input video signal is processed for conversion into a digital video signal in preparation for recording, reproduction, broadcase and so forth, it is be considered good practice to use synchronization and sample gating signals to ensure that the sampling phases of the analog input video signal are aligned in predetermined relationships. For example, when interlacing screen lines by averaging the digital video signal values for screen pixels immediately above and below each interlaced pixel, it is essential that the sampling phases for every line on the screen be aligned to ensure accurate average values. In addition, when using elements such as a noise reducer, a comb filter or the like, the sampling phases, i.e. the position of sampled data in each line of the screen must be vertically aligned. Conventionally, a time base correction circuit (TBC) has been employed to match the sampling phases during conversion of an analog video signal into a digital video signal and correct for jitter in the analog input video signal. Digital data sampling is performed on the output of the time base correction circuit. In this case, the time base correction circuit converts the analog input video signal into a pulse train indicative of a fixed number of sampled video signal values between successive pulses of a synchronizing signal contained in the input video signal, and stores the sampled video signal values in a memory. Thereafter, a read-out clock signal is produced in phase with the sampling phase of the analog input video signal. The sampled data is read out from the memory synchronously with this clock signal to re-form the analog video signal. This processing by means of the time base correction circuit has been generally accepted as an effective way of removing jitter from the analog input video signal. However, since this process always requires analog-to-digital conversion and digital-to-analog conversion, it inevitably degrades the video signal content. Also, this conventional circuit construction is complicated and bulky due to the presence of the time base correction circuit. From US-A-4053926 there is known a timing error compensator having the features of the precharacterizing parts of claims 1 and 8 and wherein, a time displacement adjustment device compensates time base errors that progressively cumulate in a time varying signal having a synchronizing information component from which the signal time base can be determined. Periodically, the synchronizing component is sampled and a representation thereof continuously generated with the component's timing relationship to the signal retained. Each continuously generated representation and the succeeding sample of the synchronizing component are compared to determine the time base error that progressively occurs in the signal between the successive samples of the synchronizing component. That signal is expanded or compressed accordingly to correct the erroneous timing. It is further known a phase-locked circuit (US-A-4 291 332), which includes a voltage controlled oscillator to supply a sampling clock signal to an analog-digital converter for converting a composite color television signal into a digital signal, and a control signal generator circuit to apply a control signal to the voltage controlled oscillator in response to the composite color television signal. The control signal generator circuit includes a burst separator for extracting a digital signal component corresponding to a color burst signal from a digital output signal from the analog-digital converter, a phase signal generator producing a reference phase signal, and a digital phase error detector circuit receiving the output signals from the burst separator and phase signal generator and supplying the voltage controlled oscillator with an output signal corresponding to the phase difference between both these input signals. However, this known phase-locked circuit has also a relative complicated construction. Furthermore a circuit for digitally encoding an analog television signal by sampling is known (US-A-4 075 656). This circuit includes an analog-to-digital encoder which receives both the analog television signal and a sampling signal generated by the circuit. A clock signal generator is utilized providing a signal of a frequency equal to a first integral multiple of a desired sampling signal frequency and to a second integral multiple of the frequency of a synchronizing component of the television signal. The clock signal is phase-locked to that synchronizing component. A frequency divider is utilized to divide the frequency of the clock signal to provide a sampling signal of a desired frequency. The resulting sampling signal frequency is a non-integral rational multiple of the horizontal sync frequency. The frequency divider is set during each horizontal blanking period to a predetermined state to provide a sampling signal whose phase is the same at the beginning of each horizontal line of the analog television signal. Vertical alignment of sampling signals on subsequent horizontal lines of the television signal results. However, this known circuit is not adapted for processing digital video signals. Therefore, it is an object of the invention to provide a method and a system for processing digital video signals including the conversion of an analog input video signal into a digital output signal which can align sampling phases without a time base correction circuit and which therefore exhibits a relativly simple circuit layout. This object is accomplished by a method as is specified in claim 1 and by a system as is specified in claim 8. Specific embodiments of the afore mentioned method are characterized in claims 2 to 7. Specific embodiments of the afore mentioned system are characterized in claims 9 to 12. The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only. In the drawings: Fih. 1 is a block diagram of the preferred embodiment of an input video signal processing system according to the present invention; Fig. 2 is a timing chart of signals produced by the components in the system of Fig. 1; Fig. 3 is a block diagram of a phase modulation circuit of the preferred embodiment shown in Fig. 1; Fig. 4 shows the waveform of the reference signal applied to the comparator in the phase modulation circuit in Fig. 3; and Fig. 5 is a timing chart illustrating one way of modulating the phase of the read-out clock signal. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, particularly to Fig. 1, the preferred embodiment of a video signal processing system in accordance with the present invention, includes a low-pass filter 1 through which an analog input video signal V IN is input to a delay circuit 2 which may comprise a delay line 2B imposing a time delay slightly longer than one horizontal line interval, and which will be referred to hereinafter as ''1H delay''. Output S₁ of the 1H delay 2, now retarded by slightly more than one horizontal line scan interval, is conducted to an analog-to-digital (A/D) converter 5 via a low-pass filter 3 and an amplifier 4. The output of the 1H delay 2 will be referred to hereafter as ''1H delay output''. The A/D converter 5 converts the received analog signal a digital video signal v₀. The 1H delay 2 includes an oscillator 2A, which produces an output having a frequency four times that of a color subcarrier frequency f sc , and a delay line 2B, which will be referred to hereafter as ''1H delay line''. ''1H delay line'' may be a charge-coupled device (CCD), for example, and is triggered by the output of the oscillator 2A. The A/D converter 5 receives a sampling clock signal CK from a sampling clock generator or generating means 10. The A/D converter 5 samples the 1H delay output S₁ in response to each sampling clock signal pulse CK. Therefore, the 1H delay output S₁ is sampled at a timing corresponding to the pulse period of the sampling clock signal. The analog-to-digital converter 5 converts the sampled analog signal level into a digital value and outputs a digital video signal V₀ in the form of a digital pulse train. The sampling clock generator 10 comprises a horizontal synchronizing signal extracting circuit 11 which includes a horizontal synchronizing signal separator circuit 12 and a burst signal gate 13. The horizontal synchronizing signal separator circuit 12 receives the analog input video signal V IN and separates a horizontal synchronizing signal H from the input video signal. On the basis of the separated horizontal synchronizing signal H, the horizotal synchronizing signal separator circuit 12 derives a gate signal S₂ having a gate period corresponding to the period of the burst signal of the input video signal V IN . The gate signal S₂ is supplied to the burst gate 13. The burst gate 13 responds to the gate signal S₂ by extracting a burst signal BUR from the input video signal V IN and supplying the extracted burst signal BUR to an input phase detector circuit 14. The waveform of the burst signal BUR is shown in (A) of Fig. 2. On the other hand, the horizontal synchronizing signal H separated by the horizontal synchronizing signal separator circuit 12 is supplied to a reference frequency signal generator 15 which comprises a phase-locked loop (PLL). The reference frequency signal generator 15 is responsive to the horizontal synchronizing signal H to produce a reference frequency signal SC having a frequency to that of the color subcarrier frequency f sc . The reference frequency signal SC is input to the input phase detector 15. The waveform of the reference frequency signal SC is seen in (C) of Fig. 2. The input phase detector 14 has a phase difference detector 21. The phase difference detector is 21 is triggered by a positive-going zero crossing of the burst signal BUR from the burst gate 13, as shown at time t₁ of Fig.2. When triggered, the phase difference detector 15 measures the phase difference Δ between the phase of the reference frequency signal SC of (C) of Fig. 2 and the burst signal BUR of (A) of Fig. 2. The phase difference detector 21 then produces a phase difference indicative signal S₃ according to the measured phase difference Δ. The phase difference indicative signal S₃ is then output to a phase-shifting circuit 22 in the input phase detector 15 at the next positive-going zero point of the burst signal BUR as shown at time t₂ of Fig. 2. The phase-shifting circuit 22 adjusts the phase of the reference frequency signal SC to an extent corresponding to the value of the phase difference indicative signal S₃ which represents the phase difference Δ between the burst signal BUR (Fig. 2A) and the reference frequency signal (Fig. 2C). The phase-shifting circuit 22 outputs a signal S₄ with the same frequency as the reference frequency signal but phase-shifted to an extent corresponding to the phase difference indicative signal value (Δ), which signal S₄ will be referred to hereafter as ''phase-shifted reference frequency signal''. As will be apparent from the above and from Fig. 2, the phase-shifted reference frequency signal S₄ is thus in phase with the burst signal BUR. The phase-shifted reference frequency signal S₄ is applied to a velocity-error detector 23. The velocity-error detector 23 checks the phase difference between the burst signal BUR and the phase-shifed reference signal S₄, which should match the phase deviation measured by the phase-shifting circuit 22 when the rotary video head is turning at precisely its rated drive speed. The velocity-error detector 23 outputs an error-indicative signal S₅ in the form of a direct-current voltage representative of the derived phase difference at time t₂. The error signal S₅ is transmitted to a sample/hold circuit 26. As set forth above, in order to detect phase of the analog input video signal V IN , the input phase detector 14 produces a phase-shifted reference frequency signal S₄ the frequency of the reference frequency signal of the reference frequency signal generator 15 and the phase of the burst signal BUR, and also produces an error signal S₅ indicative of subsequently occurring phase deviation between the burst signal BUR and the phase-shifted reference frequency signal S₄. Close inspection of analog input video signals reveals that the phases of the horizontal synchronizing signal H and the burst signal BUR most often differ. Therefore, one of the phase of the horizontal synchronizing signal H or the phase of the burst signal BUR must be taken as the phase of the input video signal. The horizontal synchronizing signal H is a rectangular waveform signal and therefore includes a plurality of frequency components. On the other hand, the burst signal BUR is an analog frequency signal. Therefore, it is easier to detect the signal phase of the burst signal BUR than that of the horizontal synchronizing signal H. This allows more accurate detection of the phase of the input video signal. For this reason, in the shown embodiment, the phase of the burst signal is preferred for use in determining the phase of the input video signal. A phase correction circuit 25 includes the sample/hold circuit 26 which is connected for input from the velocity-error detector 23 as set forth above. The sample/hold circuit 26 latches the value of the error signal S₅ at time t₃ determined on the basis of a delayed burst signal BUD extracted from 1H delay output S₁. Specifically, the 1H delay output S₁ of the 1H delay 2 is applied to a delayed burst gate 33 in a delayed signal extracting circuit 32, via an amplifier 31, as delayed input S₆. The waveform of the delayed input S₆ is illustrated in (B) of Fig. 2. The time delay imposed by the 1H delay 2 can best be understood by noting that the time points t₂ of (A) and t₁₃ of (B) correspond to the same screen pixel. Thus the delayed output S₁ is retarded by a full scan line period plus the short interval t₃-t₂ or t₁₃-t₁₂ to allow for signal processing delays. This means that in Fig. 2 the delayed burst signal BUD is derived from the preceding horizontal line while the error signal S₅ is derived from the current burst signal BUR. The delayed signal extracting circuit 32 also has a gate signal generator 34. The gate signal generator 34, which may, for example, comprise a monostable multivibrator, produces an extracting gate signal S₇ as described in detail later. The delayed burst gate 33 responds to the extracting gate signal S₇ by extracting the burst signal component BUD from the delayed input S₆ from the amplifier 31. The burst signal BUD thus extracted by the delayed burst gate 33 is supplied to the sample/hold circuit 26. As will be appreciated from Fig. 1, the extracting gate signal generator 34 is connected for input from the horizontal synchronizing signal separator circuit 12 in order to receive the horizontal synchronizing signal H. The extracting gate signal generator 34 is responsive to the leading edge of the horizontal synchronizing signal H to output the extracting gate signal S₇ having a period corresponding to the period over which the burst signal BUR appears in the input video signal. Since, the delayed input S₆ from the amplifier 31 is delayed by slightly more than one horizontal line interval, the timing of the horizontal synchronizing signal H in the 1H delay output S₁ is somewhat offset from that of the input video signal. Therefore, the extracting gate signal generator 34 delays by this known offset the horizontal synchronizing signal H extracted from the input video signal V IN to ensure that all of the delayed burst signal BUD can be extracted from the delayed input S₆. The delayed burst signal BUD from the delayed burst gate 33 is also input to a phase difference detector 36 which is also connected to the reference frequency signal generator 15 to receive the reference frequency signal SC. The phase difference detector 36 compares the signal phases of the delayed burst signal BUD from the delayed burst gate 32 and the reference frequency signal SC from the reference frequency signal generator 15 to derive the phase difference therebetween. The phase difference detector 36 produces a phase-shifting control signal S₉ having a value indicative of the phase difference between the delayed burst signal BUD and the reference frequency signal. The phase difference detector 36 operates slightly differently than the phase difference detector 21 described above. At the second positive-going zero-crossing point of the delayed burst signal BUD, the phase difference detector 36 compares the phases of the delayed burst signal BUD and the reference frequency signal SC. The phase-shifting control signal S₉, which corresponds the phase difference indicative signal S₃ of the phase difference detector 21, is then output immediately to a phase-shifting circuit 37. The phase shifting circuit 37 receives the phase-shifting control signal S₉ from the phase difference detector 36 and shifts the phase of the reference frequency signal SC to an extent corresponding to the value of the phase-shifting control signal S₉ from the phase difference detector 36. The phase-shifting circuit 37 outputs a phase-shifted frequency signal S₁₀ which is in phase with the delayed burst signal BUR. The phase-shifted frequency signal S₁₀ from the phase-shifting circuit 37 is supplied to a phase modulation circuit 38. The phase modulation circuit 38 also receives the output S₁₁ of the sample/hold circuit 26. The phase modulation circuit 38 adjusts the phase of the phase-shifted frequency signal S₁₀ from the phase-shifting circuit 37 to an extent determined by the output S₁₁ of the sample/hold circuit 26, which is indicative of the phase difference between the burst signal BUR and the phase-shifted reference signal S₄. The adjustment of the signal phase of the phase-shifted frequency signal S₁₀ is performed gradually and proportionally over a period corresponding to one horizontal line interval. The phase modulation circuit 38 outputs the resulting frequency signal phase-adjusted as described above, for use as a clock signal S₁₂. The waveform of the clock S₁₂ from the phase modulation circuit 38 is shown in (G) of Fig. 2. Although not noticeable in Fig. 2(G), the phase of each successive pulse of the clock S₁₂ varies slightly, as will become clear later. Fig. 3 shows the detailed structure of the phase modulation circuit 38. The phase modulation circuit 38 includes a level comparator 51. A reference signal S₂₁ is applied to the inverting input (-) of the level comparator 51. The reference signal is a ramp signal having a waveform as shown in Fig. 4. The reference signal S₂₁ starts at a value V S11 corresponding to the level of the output S₁₁ of the sample/hold circuit 26, at time t₃ and then drops linearly to zero over a period equal to one horizontal scan line interval (1H), as shown in Fig. 4. The phase modulation circuit 38 also has a sawtooth signal generator 52 which receives the phase-shifted frequency signal S₁₀ from the phase-shifting circuit 37, the waveform of which is illustrated in (A) of Fig. 5. The sawtooth signal generator 52 outputs a sawtooth signal S₂₂ with the same period T R as the phase-shifted frequency signal S₁₀. The waveform of the sawtooth signal S₂₂ has been illustrated in (B) of Fig. 5. The sawtooth signal S₂₂ is input to the non-inverting input (+) of the level comparator 51. The level comparator 51 outputs a comparator signal S₂₃ which remains HIGH for the period during which the signal level of the sawtooth signal S₂₂ exceeds the reference signal level S₂₁. A typical waveform of the comparator output S₂₃ is illustrated in (C) of Fig. 5. The comparator signal S₂₃ is fed to a waveform shaping circuit 53. When triggered by the leading edge of the comparator signal S₂₃ to the waveform shaping circuit 53 produce a pulse with a predetermined pulse width. The output train of these pulses forms clock signal S₁₂ of (D) of Fig. 5. It should be appreciated that the period T R ' of the clock S₁₂ may differ slightly from that of the reference frequency signal SC of the reference frequency signal generator 15, the difference being dependent on the value of signal S₁₁ and on the position of each pulse in the horizontal scan line. In the shown embodiment, the pulses falling at the center of each horizontal line will always be in precise alignment vertically, and due to the linear nature of the sawtooth signal S₂₂ and the reference signal S₂₁, the phase error will be divided symmetrically to either side of the screen. This maximizes image quality by limitting the maximum vertical offset and by providing the best accuracy in the center of the image. In the circuit structure of Fig. 3, the phase at which the sawtooth signal level first exceeds the reference level defined by the reference signal S₂₁ advances as the reference level of the reference signal S₂₁ drops. Therefore, the period T R ' of the clock S₁₂ varies gradually and continuously at a rate depending upon the rate of decrease of the reference signal determined by the value of the signal S₁₁. As will be appreciated herefrom, since the phase modulation technique set forth above covers an entire cycle of the ramp signal S₂₁, i.e. one horizontal line interval, over which the value V S11 represented by the output S₁₁ of the sample/hold circuit 26 is uniformly distributed, the phase error for each horizontal scan line is divided equally among all of the pulses of the clock S₁₂ for that scan line. Thus, no matter how great the phase error, each scan line will have the same number of pixels or color clock pulses S₁₂, and the horizontal offset between pixels of adjoining scan lines will be reduced to a negligible level. Returning to Fig. 1, the clock S₁₂ thus produced by the phase modulation circuit 38 is supplied to a multiplier 42 through a temperature compensation circuit 41. According to the shown embodiment, the multiplier 42 multiplies the clock frequency by 4 to generate a frequency 4f sc four-times higher than the frequency of the reference frequency signal. The output of the multiplier 42 serves as sampling clock CK for the A/D converter 5. The waveform of the sampling clock CK is shown in (H) of Fig. 2. The temperature compensation circuit 41 compensates for phase variation of the sampling clock CK due to temperature variations. The temperature compensation circuit 41 has a divider 43 which divides the frequency of the sampling clock CK by four to match the frequency of the clock S₁₂ of the phase modulation circuit 38. The divided output of the divider 43 is fed to a phase difference detector 44 which also receives the clock S₁₂ from the phase modulation circuit 38. The phase difference detector 44 compares the phases of the clock S₁₂ and the divided output from the divider 43 and outputs a phase difference indicative signal S₂₅ to a phase-shifting circuit 45 interposed between the phase modulation circuit 38 and the multiplier 42. The phase difference indicative signal S₂₅ has a value representative of the phase difference between the divided output from the divider 43 and the clock S₁₂. Therefore, by shifting the phase of the clock S₁₂ to an extent equal to the phase difference between the divided output and the clock S₁₂, the phase of the clock S₁₂ is compensated for possible temperature-dependent variations thereof. The operation of the preferred embodiment of input video processing system according to the present invention will be described with reference mainly to Figs. 1 and 2. At the time t₁ in Fig. 2, the phase of the analog input video signal V IN , i.e., the phase of the burst signal BUR extracted by the burst gate 13 in response to the gate signal S₂ from the horizontal synchronizing signal separator circuit 12, is compared with the reference frequency signal SC of the reference frequency signal generator 15, which has the same frequency as the color subcarrier frequency f sc , by the phase difference detector 21 of the input phase detector 14. The phase difference indicative signal S₃ output by the phase difference detector 21 has a value corresponding to the phase difference between the input video signal phase and the reference frequency signal phase. Phase-shifting operation of the phase-shifting circuit 22 in the input phase detector 14 is mediated by the phase difference indicative signal S₃. The phase-shifting circuit 22 shifts the signal phase of the reference frequency signal SC to the extent indicated by the phase difference indicative signal S₃ and outputs the phase-shifted reference frequency signal S₄, as shown in (D) of Fig. 2, which should be in phase with the burst signal BUR of (A) of Fig. 2. The phase-shifted reference frequency signal S₄ is applied to the velocity-error detector 23 at the time t₂. The velocity-error detector 23 measures any existing phase difference between the phase-shifted reference frequency signal S₄ and the burst signal BUR and produces an error indicative signal S₅ having a value indicative of the derived phase difference. The velocity-error detector 23 outputs the error indicative signal S₅ to the sample/hold circuit 26. At the time t₃, the phase of the delayed burst signal BUD, which is extracted from the 1H delayed output S₁, is compared with the reference frequency signal SC to derive the phase difference therebetween. The phase of the reference frequency signal is shifted by the phase-shifting circuit 37 to an extent equal to the phase difference between the delayed burst signal BUD and the reference frequency signal SC in order to match phase with the delayed burst signal. The phase-shifted frequency signal S₁₀ is applied to the phase modulation circuit 38. At the same time, the output S₁₁ representing the value held in the sample/hold circuit 26 is also applied to the phase modulation circuit 38. As set forth above, the phase modulation circuit 38 distributes the phase error indicated by the output S₁₁ from the sample/hold circuit 26 among the pulses of the clock S₁₂ within one horizontal line interval. Therefore, irrespective of the magnitude of the holding output S₁₁, a predetermined number of pulses of clock S₁₂ can be output in each horizontal line interval. The clock S₁₂ is increased in frequency by four times by the multiplier 42 and output as sampling clock CK. The sampling clock CK is output to the A/D converter. Since the clock S₁₂ of the phase modulation circuit 38 always has a constant, predetermined number of pulses within each horizontal line interval, the sampling clock CK will also always have a constant, predetermined number of pulses. The A/D converter 5 is triggered by each pulse of the sampling clock CK to sample the 1H delay output S₁ and output the sampled value as digital video signal V₀. As will be appreciated herefrom, the 1H delay output S₁ will be sampled at a predetermined constant number of sample points during each horizontal scan line. According to the shown embodiment, conversion of the analog input video signal V IN into digital video signal V₀ is performed at a constant number of sample point within each horizontal line interval, even when the analog input video signal is subject to significant jitter. In addition, the phase offset between corresponding sampling points of each horizontal line is minimized, so that the vertical positions on a screen of corresponding sample points will remain in close alignment. While the present invention has been disclosed in terms of the specific embodiment as illustrated in the drawings, the invention can be embodied in many ways without departing from the principle of the invention and the shown embodiment can be modified as necessary or desired to accomplish the same or similar effects. For example, although the phase modulation circuit in the preferred embodiment distributes the phase error linearly among the pulses of the clock signal, distribution rate of the phase error need not be linear. It is essential only that the distribution be consistent for each horizontal line. Therefore, as long as the phase error is distributed consistently for each horizontal line and as long as the phases of corresponding sample points can be uniform, any distribution technique may be used to embody the present invention. Also, although the shown embodiment has been disclosed in terms of analog input video signal processing, in which the clock signal is generated with reference to the 1H delay output produced by delaying the input video signal for one horizontal line interval, it should not be considered essential to have a 1H delay: the delay can be of any convenient length.

Claims: A method for processing digital video signals including conversion of an analog input video signal (V IN ) into a digital output signal (V₀) by sampling analog video signal values in response to sampling signals (CK), measuring the phase of the analog input video signal (V IN ) at a given timing relative to a reference signal (SC) phase and deriving the phase difference therebetween; delaying the input video signal (V IN ) for a given period to obtain a delayed video signal (S₁); measuring possible spurious phase error in the input video signal by measuring the rate of change of the phase difference between the input video signal and said reference signal (SC); producing a predetermined number of clock pulses (S₁₂) in each horizontal line interval, said clock pulses being derived from said reference signal (SC) by shifting the phase of every pulse of said reference signal by said measured phase difference so as to be in phase with said video input signal (V IN ) and shifting the phase of each pulse of said phase-shifted reference signal (S₄) to an extent derived by distributing the measured spurious phase error among the predetermined number of clock pulses (S₁₂) for each horizontal line, deriving said sampling signal (CK) from said clock pulses (S₁₂); characterized in that the shifting of the phase of every pulse of said reference signal (SC) is performed by phase modulation process including the steps of generating rising sawtooth signals (S₂₂) having a duration corresponding to a pulse period of said reference signal (SC), generating a threshold signal (S₂₁) having an initial level corresponding to said spurious phase error and gradually decreasing to zero over the period of one horizontal line interval, comparing the level of each sawtooth signal (S₂₂) with the level of said threshold signal (S₂₁) and producing a clock pulse (S₁₂) with a fixed pulsewidth each time said sawtooth signal level increases beyond the level of said threshold signal (S₂₁); the signal level of said threshold signal (S₂₁) is so set relative to the signal level of said sawtooth signal (S₂₂) that clock pulses (S₁₂) produced at the center of each horizontal line interval are precisely in phase with the input video signal. The method as set forth in claim 1, characterized in a step of extracting a horizontal synchronizing signal (H) defining the beginning of each horizontal line interval. The method as set forth in claim 1 or 2, characterized in that said reference signal is a pulse signal having the same frequency as a color subcarrier wave of the video input. The method as set forth in any one of claims 1 to 3, characterized in that said step of measuring the phase of said video input signal (V IN ) includes a step of extracting a burst signal (BUR) from said analog input video signal (V IN ), the phase of the burst signal (BUR) being taken as representative of the phase of said input video signal (V IN ). The method as set forth in claim 4, characterized in that said given timing for detecting the phase difference between said burst signal phase and said reference signal phase is determined by the phase of said burst signal (BUR). The method as set forth in any one of claims 1 to 5, characterized in the steps of: measuring the phase of said delayed video signal (S₁), comparing said phase of said delayed video signal (S₁) with said reference signal (SC) at said given timing and deriving the phase difference therebetween; and shifting the phase of said reference signal (SC) to an extent matching the derived phase difference between said delayed video signal (S₁) and said reference signal (SC) so as to match the phase of said reference signal (SC) with that of said delayed video signal (S₁) and so produce a phase-shifted reference signal (S₁₀). The method as set forth in claim 6, characterized in that said step of shifting the phase of every reference signal pulse by said measured phase difference is performed prior to the step of shifting the phase of every pulse of said reference signal (SC) to said extent derived from said measured spurious phase error. A system for processing digital video signals including means for converting of an analog input video signal into a digital output signal by sampling of analog video signal values in response to sampling signals; a first detector measuring the phase of the analog input video signal at a given timing relative to a reference signal (SC) phase and deriving the phase difference therebetween; a delay circuit (2) delaying the input video signal for a given period to obtain a delayed video signal; an error detector (23) measuring possible spurious phase error in the input video signal by measuring the rate of change of the phase difference between the input video signal and said reference signal (SC); a clock pulse generator means (37, 38) for producing a predetermined number of clock pulses in each horizontal line interval, said clock pulses being derived from said reference signal (SC) by shifting the phase of every pulse of said reference signal (SC) by said measured phase difference so as to be in phase with said video input signal and shifting the phase of each pulse of said phase-shifted difference signal to an extent derived by distributing the measured spurious phase error among the predetermined number of clock pulses for each horizontal line, sampling signal generator means (10) for deriving said sampling signal (CK) from said clock pulses (S₁₂), characterized by a second detector (36) measuring the phase of said delayed video signal, comparing said phase of said delayed video signal with said reference signal (SC) at said given timing and deriving the phase difference the phase difference therebetween; a phase-shifting circuit (22) shifting the phase of said reference signal (SC) to an extent matching the derived phase difference between said delayed video signal and said reference signal so as to match the phase of said reference signal (SC) with that of said delayed video signal and so produce a phase-shifted reference signal (S₄); said clock pulse generating means generates rising sawtooth signals (S₂₂) having a duration corresponding to the pulse period of said reference signal (SC), and a threshold signal (S₂₁) having an initial level corresponding to said spurious phase error and gradually decreasing to zero over the period of one horizontal line interval, and compares the level of each sawtooth signal (S₂₂) with the level of said threshold signal (S₂₁) and produces a clock pulse (S₁₂) with a fixed pulsewidth each time said sawtooth signal (S₂₂) level increases beyond the level of said threshold signal (S₂₁). The system as set forth in claim 8, characterized in a horizontal synchronizing signal extracting circuit (12) extracting a horizontal synchronizing signal (H) defining the beginning of each horizontal line interval. The system as set forth in claim 8 or 9, characterized in a reference signal generator (15) generating said reference signal (SC) in the form of a train of pulses at the same frequency as a color subcarrier wave. The system as set forth in anyone of claims 8 to 10, characterized in a burst gate (13) receiving a burst gate signal (S₂) from said horizontal synchronizing signal extracting circuit (12) and extracting a burst signal (BUR) from said analog input video signal (V IN ), the phase of said burst signal (BUR) being taken as representative of the phase of said input video signal (V IN ). The system as set forth in claim 11, characterized in that said given timing for detecting the phase difference between said burst signal phase and said reference signal phase is determined by the phase of said burst signal (BUR).

Inventors: TATAMI, MITSUSHIGE C/O SONY CORPORATION
Applicants: SONY CORPORATION
Application year: 1985
Main classification: H04N9
IPC codes: H04N9, G11B20, H04N5