Patent GB-1239297-A (GB)
IPC: H04N9/475
Filed: 1968-10-10
Abstract: 1,239,297. Automatic phase control. PYE Ltd. 8 Oct., 1969 [10 Oct., 1968], No. 48054/68. Heading H3A. In a method of providing synchronization between locally and remotely generated synchronizing waveforms for a colour television system, signals at line frequency are employed to lock an additional oscillator at twice line frequency, changeover of lock between local and remote control being allowed to occur only when phase coincidence is achieved between signal at line frequency generated from the output of a local master oscillator (" Phantom H ") and line frequency signal from the remote station (" Remote H "), and then only during the field blanking period. The phase coincidence is obtained by advancing the phase of the Phantom H waveform when the phase difference between a pulse of Remote H and the succeeding pulse of Phantom H is greater than 180 degrees, or by retarding the phase of the Phantom H waveform when the phase difference referred to is less than 180 degrees. Advance or retard is effected by addition or subtraction of pulses from a waveform at the input of a frequency divider circuit in the local frequency converter, and occurs only during the field blanking period. The local master oscillator comprises colour subcarrier crystal oscillator 1, Fig. 1, whose frequency can be varied over a small range by means of a voltage dependent capacitor: on Local lock the control potential is supplied from potentiometer 2 via Local/Remote switch contacts S1a. The output from oscillator 1 is applied to frequency converter 3 which provides an output at twice line frequency (2H). Waveform generator 4 is driven from an auxiliary voltage controlled sine wave oscillator 5 of nominal frequency 2H, via clipper 6 converting the waveform to square wave, and monostable multivibrator 7. The frequency of oscillator 5 is controlled by discriminator 9, one input of which is derived from clipper 6 via divide-bytwo circuit 8 and the other from pulse generator 10: pulse generator 10 is fed in Local and Remote lock with Phantom H and Remote H waveform respectively, from the output of an electronic switch A. Phantom H waveform is produced from the output of frequency converter 3 by means of a divide-by-two circuit 11 and is converted to positive-going pulses by a pulse generator 40. Remote H waveform is obtained from incoming video signal by means of sync. separator 12. To permit mixing of local and remote signals the corresponding sync. signals must be in phase, and the phase of the Local H waveform, derived from waveform generator 4 by means of local sync. separator 14, is compared with the output from pulse generator 10 in a phase discriminator 13. The output of pulse generator 10 is applied to a shaper circuit 15 which produces a train of pulses having ramp-type leading edges, which are applied to the second input of discriminator 13, the output of which is applied as variable bias to monostable multivibrator 7: this is triggered by the leading edges of the square waves from clipper 6, but the width of the pulses produced depends on the bias from discriminator 13, so the trailing edges of the pulses are advanced or retarded by variation of the bias. Waveform generator 4 is triggered by these trailing edges, so varying the phase of the Local H pulses. To lock local subcarrier oscillator 1 to the remote subcarrier, a signal from it is applied to discriminator 16, to a second input of which is applied the colour burst component of the incoming video signal, separated out in remote sync. separator 12: low-pass filter 17 ensures that the control signal obtained from discriminator 16 corresponds to the mean phase of the burst signal. This control signal is fed via contacts S1a, in Remote position, to the control input of oscillator 1. Discriminator 16 also produces a signal corresponding to the line-by-line alternation in phase of the burst signals and this is separated out by filter 18. Change from local to remote lock.-Electronic switch A comprises AND gates 19, 20 whose outputs are applied to OR gate 21: Phantom H and Remote H waveforms are applied to first inputs of gates 19, 20 respectively and to the second inputs are fed signals in antiphase from bi-stable 22. Reset signals are provided by a phase comparator and coincidence detector unit B and are directed to the L or R inputs of bistable 22 by relay contacts RL1/1: relay RL1 is shunted by capacitor C and is energized through contacts S1c of Local/Remote switch S1 which always completes changeover before RL1 operates, so that electronic switch A cannot be reversed while Phantom H waveform is still being applied to phase detector unit B. Unit B provides reset signal to electronic switch A only when phase coincidence has been established between Local H and Remote H waveforms and also between local and remote burst axis gate waveforms, and after reversal of switch S1a has locked the frequency of oscillator 1 to Remote H. Phase coincidence is achieved by modifying the frequency of oscillator 1. Bi-stable 24 is fed with Remote H pulses, its output being a square wave of frequency H/2: it is reset by signal dependent upon the line-by-line alteration of phase of incoming colour signal, from clipper 23 coupled by filter 18 to discriminator 16, the bi-stable 24 producing two outputs a, Fig. 3.3 comprising reconstituted remote axis gate waveform and b, Fig. 3.4 the inverse. These waveforms are applied to the first inputs of AND gates 25, 26 respectively. In waveform generator 4, divide-by-two circuit 27 driven at frequency H produces the Local colour axis gate waveform, Fig. 3.6: pulse generator 28 produces negative pulses when triggered by the positive-going edges of waveform 3.6, which are inverted in circuit 29 to give positive pulses, Fig. 3.7. The negative-going edges of these trigger pulse generator 30 to produce 2 Ásec. pulses, Fig. 3.8, which are applied to the second inputs of gates 25, 26. If the remote axis gate waveform leads the local waveform, pulses are passed by gate 25 and gate 26 is shut, while if the remote waveform lags the opposite occurs: the outputs of gates 26, 25 are respectively connected to set and reset inputs of bi-stable 31 which is set by a pulse from gate 26 to cause phase of the Phantom H waveform to advance, and reset by a pulse from gate 25 to cause retardation. To prevent a condition near coincidence when both gates would pass pulses, the remote axis gate waveform, Fig. 3.3, is applied to pulse generator 32 which is triggered by the positive-going leading edges to produce negative-going pulses of 15 Ásec. duration, Fig. 3.5, which are applied to the third inputs of gates 25, 26 to prevent gate 25 passing the 2 Ásec. pulses if they occur during these periods. To avoid disturbance caused by operation during the period of 9 lines at the beginning of each field, when colour burst signals are not transmitted, a blanking pulse is applied to the fourth inputs of gates 25, 26 from a pulse generator 34 triggered by remote vertical synchronizing pulses derived from remote sync. separator 24. To provide a reset signal for electronic switch A, the 10 Ásec. pulses derived via pulse generator 28 and inverter 29 from the local burst axis waveform are fed to one input of five input positive AND gate 35, to a second input is fed the 15 Ásec. pulses derived from the remote axis gate waveform view pulse generator 32, after inversion in inverter 33. To a third input is fed the Local H waveform and to a fourth either Phantom H or Remote H waveform, as selected by switch contacts S1b: local vertical synchronizing signal, separated out in local sync. separator 14 is fed to the fifth input. To achieve coincidence of the local and remote waveforms, the frequency of the Phantom H waveform is increased or decreased by modification of the effective overall ratio of frequency converter 3 which may be of the type disclosed in Specification 1,217,699, containing a frequency divider fed with pulses from subcarrier oscillator 1: to advance the phase of the Phantom H waveform pulses are added at the divider input, and to retard the phase pulses are subtracted, under control of logic circuit 36, which is gated by a local vertical blanking waveform from generator 4 so that the divider output is modified only during vertical blanking periods. Frequency drift is required to commence when the condition of switch S1 is changed and to cease when coincidence is attained, contacts S1d being connected across positive and negative supplies to produce an impulse triggering pulse generator 37 each time switch S1 is reversed. The output of generator 37 is fed to " set " input terminals of bi-stable 38 so that in logic " 1" condition the frequency of converter 3 is modified: the " reset " input of bi-stable 38 is fed from a positive OR gate 39. Each time switch S1 is operated, bi-stable 38 is set to " 1 " condition and frequency drift commences: when coincidence is attained, electronic switch A operates and a positive pulse is transmitted via OR gate 39 to reset bi-stable 38 and terminate frequency drift. The output of bi-stable 31 which determines whether the phase of the Phantom H waveform is to be advanced or retarded is connected to logic unit 36 to cause addition or subtraction of pulses at the input of frequency converter 3. The Specification also gives a detailed description of logic unit 36 (Figs. 4, 5, not shown). If a momentary loss of remote signals occurs, upon return the local synchronizing unit may no longer be accurately in phase: since disturbance of the picture has already occurred it is not necessary to establish phase identity between local and remote signals before resuming remote lock, and a circuit is provided to bring the local sync. generator back into phase in these conditions. Remote H/2 from discriminator 16 is fed via AND gate 50 and switch contacts S1e to reset bi-stable 27 generating local axis gate waveform: gate 50 receives