Video signal phase and frequency correction using a digital off-tape clock generator

Claim 2

A television signal processing system, wherein color video signal has a subcarrier frequency signal, and sync and color burst components, comprising: means for digitizing said sync and color burst signal components and for supplying the digitized components at an output thereof; said digitizing means having a clock input for receiving a sampling clock of predetermined frequency; signal processing means, coupled to the output of said digitizing means, for measuring the phase of the digitized color burst component relative to the phase of the sampling clock; and said signal processing means including a closed feedback loop coupled from the phase measuring means back to the clock input of said digitizing means, and further including means for applying phase and frequency corrections to the sampling clock in response to the relative phase measurement to match the sampling clock to the color video signal.

Claim 3

A television signal processing system as in claim 1, wherein said means for digitizing said sync and color burst signals is an analog-to-digital converter coupled to directly receive the color video signal.

Claim 4

A television signal processing system as in claim 1, wherein said signal processing means includes: a digital signal processor, coupled to the digitizing means for generating a frequency control word indicative of the difference in phase between the digitized color burst component and the sampling clock; and a digital frequency synthesizer, coupled to said digital signal processor, for generating the sampling clock with a phase adjusted in response to the phase measurement, and with the frequency adjusted in response to said frequency control word.

Claim 5

A television signal processing system as in claim 3, wherein said digital signal processor includes: digital phase subtractor means for supplying the phase difference between two consecutive digitized color bursts; and digital frequency accumulator means coupled to the phase subtractor means for supplying said frequency control word corrected on a line-by-line basis to said digital frequency synthesizer.

Claim 6

A television signal processing system as in claim 3, further including; phase calculating means, coupled to the digitizing means, for supplying sampling clock-to-color burst phase errors to the digital signal processor for measurement thereof.

Claim 7

A television signal processing system as in claim 5 wherein: said phase calculating means supplies the clock-to-burst phase errors to the digital frequency synthesizer on a line-by-line basis to effect phase correction of the sampling clock.

Claim 8

A television signal processing system as in claim 6 wherein the digital frequency synthesizer includes: digital phase accumulator means for maintaining the current phase of the digital frequency synthesizer; and means coupled to the digital phase accumulator means for transferring the phase thereof into a clocking edge having a corresponding phase to provide the adjusted sampling clock to the digitizing means.

Claim 9

In a television recording and playback system wherein input video signals are processed, said signals having a nominal subcarrier frequency signal and color burst and horizontal sync components, a digital clock generator circuit comprising: means for selectively delaying said input video signals to provide a delayed late video signal and an undelayed early video signal; a converter for sampling said delayed and undelayed video signals at a multiple frequency of said subcarrier in response to a sampling clock, and for converting said delayed and undelayed video signals to corresponding digital signals; a digital signal processsor, coupled to receive the corresponding digital signals from said converter, for providing a phase error signal for adjusting the phase of the sampling clock while providing a digital frequency control word for adjusting the frequency of the sampling clock a digital frequency synthesizer, coupled to said processor, for supplying the sampling clock to the converter with its phase and frequency adjusted in response to said phase error signal and said frequency control word.

Claim 10

In a television recording and playback system wherein input video signals are processed, said signals having a nominal subcarrier frequency signal and color burst and horizontal sync components, a digital clock generator circuit comprising: means for selectively delaying said input video signals to provide a delayed late video signal and an undelayed early video signal; a converter for sampling said delayed and undelayed video signals at a multiple frequency of said subcarrier in response to a sampling clock, and for converting said delayed and undelayed video signals to corresponding digital signals; a digital signal processsor, coupled to receive the corresponding digital signals from said converter, for providing a phase error signal for adjusting the phase of the sampling clock while providing a digital frequency control word for adjusting the frequency of the sampling clock a digital frequency synthesizer, coupled to said processor, for supplying the sampling clock to the converter with its phase and frequency adjusted in response to said phase error signal and said frequency control word.

Claim 11

A circuit as in claim 8, including a video switch coupled between said delaying means and said converter, for switching between said delayed and undelayed signal at predetermined times.

Claim 12

A circuit as in claim 9, including a bandpass filter, 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 in phase.

Claim 13

A circuit as in claim 9 wherein, said converter supplies consecutive digitized color bursts on a line-by-line basis; and said digital signal processor includes a clock-to-burst phase calculator coupled to receive consecutive digitized color bursts and for supplying error signals representing phase error between the sampling clock and zero crossings of said burst signal component.

Claim 14

A circuit as in claim 11, wherein said digital signal processor further includes: a burst phase subtractor coupled to said clock-to-burst phase calculator for supplying the phase error between consecutive digitized color bursts; and a frequency accumulator coupled to the burst phase subtractor for adjusting the frequency of the digital frequency control word in response to the subtracted phase error.

Claim 15

A circuit as in claim 12, wherein said frequency accumulator generates an adjusted frequency control word on a line-by-line basis for application to said frequency synthesizer.

Claim 16

A circuit as in claim 12, wherein said burst phase subtractor comprises: a storage register controlled by a horizontal sync pulse clock; and a subtractor coupled to said register and operating over a 360° range for providing a clock phase error across a horizontal line.

Claim 17

A circuit as in claim 14, further including a constant scaler, connected to said subtractor, to provide a clock frequency error from the subtracted phase error.

Claim 18

A circuit as in claim 12, wherein said frequency accumulator comprises an adder and a register timed by a horizontal sync clock, said adder and register connected in a closed loop for generating the frequency control word.

Claim 19

A circuit as in claim 11, wherein said digital frequency synthesizer includes: a phase accumulator coupled to the digital signal processor, for supplying a binary number representing 360° of phase, and which phase changes by an amount determined by said digital frequency control word.

Claim 20

A circuit as in claim 17, wherein said frequency synthesizer includes a sine lookup program read only memory for generating a sine waveform substantially without harmonics and having a phase representing the phase of said phase accumulator.

Claim 21

A circuit as in claim 18, including: a digital-to-analog converter coupled to said read only memory; and a low pass filter coupled to said digital-to-analog converter and having a cutoff frequency whereby a pure fundamental sine wave is produced.

Claim 22

A circuit as in claim 8, including a low pass filter, connected in a second channel to said analog-to-digital converter, for filtering noise from said horizontal sync component.

Claim 23

A circuit as in claim 20, including means for selecting burst zero crossings that are coincident with the horizontal sync pulses, for generating a horizontal write pulse to achieve phase adjustment for the horizontal picture position.

Claim 26

A method as in claim 22 wherein: the step of digitizing includes, supplying two consecutive digitized color bursts corresponding to consecutive color burst components to provide late and early digitized color bursts; and the step of measuring includes, subtracting the phase errors between the late and early digitized color bursts to provide the phase error across a line, and generating a digital frequency control word corresponding to said frequency error correction signal in response to the phase error across the line.

Claim 27

A method as in claim 23 wherein the step of removing inlcudes: adjusting the phase of the sampling clock by correcting the clock-to-burst phase error at the beginning of each line; and adjusting the frequency of the sampling clock on a line-by-line basis in response to the digital frequency control word.

Claim 28

A method as in claim 24 including: operating the sampling clock at a frequency that is a multiple of said subcarrier frequency; and digitizing the television signal, and the sync and color burst components with the adjusted sampling clock which is locked to the color burst component of the television signal.