Switching sequence detector for PAL color television

Abstract

A time-base correcting system primarily adapted for use in video tape recording equipment and providing synchronous lockup and time base corrected operation of the video-tape recorder in the playback mode. The system includes circuitry receiving phase alternating line (PAL) composite television signals and which determines the burst switching sequence of the recorded signal before stable lockup is achieved.

I claim: 
     
       1. A system for developing a color burst phase switching sequence signal from a phase alternate line color television signal comprising in combination: means for extracting the color burst from each line of said television signal, said color bursts reversing phase every other one of said lines;   means for generating a reference signal in predetermined time relation to the horizontal synchronizing pulse of each line of said television signal, said reference signal having a constant pulse with respect to said synchronizating pulses;   means for comparing the phases of said color bursts and said reference signal to generate a burst phase representative signal varying in amplitude at half the color burst phase switching rate of said television signal; and   means for doubling the frequency of said burst phase representative signal to thereby produce a burst-switching sequence signal at the color burst phase switching rate of said television signal.   
     
     
       2. A system in accordance with claim 1, further defined by the frequency doubling means including differentiator and rectifier means coupled to receive said burst phase representative signal and generate a train of unidirectional pulses having a repetition rate numerically equal to said color burst phase switching rate and conversion means coupled to said differentiator and rectifier means for converting said train of pulses to a continuous waveform at said color burst phase switching rate. 
     
     
       3. A system in accordance with claim 2, in which the conversion means includes a shaping network for generating a continuous square wave at said color burst phase switching rate. 
     
     
       4. A system in accordance with claim 3, further defined by a switching circuit receiving said continuous square wave and synchronizing signal, the conductive and nonconductive states of the switching circuit corresponding to the polarity relationship of the edges of the respective signals. 
     
     
       5. In a signal reproducing system providing a color television signal containing phase-related components of a subcarrier having a selected nominal frequency wherein a color burst signal phase-related component is included whose phase alternates in a known sequence between two selected phases at a rate of one-half the horizontal sync pulse frequency with its phase alternated each reproduced line, the combination comprising, means for extracting the phase alternating color burst signal from the reproduced color television signal, means providing a stable reference signal at the nominal subcarrier frequency, means responsive to the phase alternating color burst signal for providing a signal indicative of the known phase switching sequence and rate of the color burst signal included in each reproduced line of the reproduced color television signal, means coupling the phase switching indicative signal to change the phase of the stable reference signal to that of the selected phases, and means for comparing the phase of the stable reference signal to the actual reproduced phase of the reproduced phase alternating color burst signal to provide an error signal representative of a difference in the actual phases of the compared signals. 
     
     
       6. A system in accordance with claim 5 further comprising means for adjusting the phase of the reproduced color television signal in response to the error signal. .Iadd. 7. A system for developing a color burst phase switching sequence signal from a phase alternate line color television signal comprising in combination: means for extracting the color burst from each line of said television signal, said color bursts reversing phase every other one of said lines;   an oscillator responsive to a signal in predetermined time relation to the horizontal synchronizing pulse of each line of said television signal to be gated to generate a discrete reference signal at the color burst frequency having a constant phase with respect to said synchronizing pulses;   means for comparing the phases of said color bursts and said reference signal to generate a burst phase representative signal varying in amplitude at half the color burst phase switching rate of said television signal; and   means for doubling the frequency of said burst phase representative signal to thereby produce a burst switching sequence signal at the color burst phase switching rate of said television signal. .Iaddend..Iadd. 8. A system in accordance with claim 7 further defined by means for extracting a burst gating pulse from each line of said television signal in predetermined time relation to the horizontal synchronizing pulse of each line of said television signal, said burst gating pulse coupled to gate said oscillator..Iaddend.