GB1251235A - - Google Patents

Abstract

1,251,235. Television. TELEMATION Inc. 23 Oct., 1968 [23 Oct., 1967], No. 47387/68. Heading H4F. A television timing pulse generator comprises a clock oscillator the frequency of which is several times greater than twice the line frequency, the leading and trailing edges of all the timing pulses being precisely clock derived by division through digital circuitry to produce the required vertical drive, horizontal drive, composite sync., mixed blanking output pulses, and in colour television burst gate pulses. As shown, Fig. 1, the generator comprises a high frequency clock oscillator 4, preferably crystal controlled, the frequency of which for NTSC standards comprises four times the 3À58 MHz colour subcarrier frequency. The clocking frequency is divided by four in counter 8 to derive the colour subcarrier and divided by seven in counter 6 and fed via a horizontal sync. lock controlled oscillator 26 (see later) to a series of counters 16 comprising divide-by-sixty-five counter 18, divide-by-three counter 20, divideby-seven counter 22, and divide-by-twenty-five counter 24. For monochrome operation the colour section 2 may be replaced by or suitably switched to high-frequency clocking oscillator 28. Multiple output pulses derived from within individual counters 18, 20, 22 and 24 provide logic 32 with accurately clock-timed pulse signals i.e. a plurality of waveforms (Fig. 3, not shown) are passed through separate paths in multiple connections 42, 44 and 46. Logic 3 digitally translates the signals into vertical drive, horizontal drive, composite sync., and mixed blanking waveforms and a burst gate pulse train. The counters of set 16 comprise " JK " flip-flops (Fig. 2, not shown) and the logic 32 comprises a plurality of NOR gates (Figs. 2 and 4, not shown). To lock the output pulses of the synchronizing generator in frequency and in phase with a remote source of composite sync. signal e.g. for creating special effects wherein two or more images are keyed or superimposed to produce a common mixed picture, the high frequency clock pulses from clock 28, Fig. 13, are fed to a phase comparator 430 in sync. lock system 26, together with the continuous pulse-train information from voltage controlled oscillator (VCO) 436 which is also fed to the counter set 16 as in Fig. 1, the output of phase comparator 430 being fed via switch 428 and low-pass filter 434 to control VCO 436. In addition a second phase comparator 438 compares the horizontal pulse train information from a remotely located generator on line 440 with the output of the local sync. generator on line 418, and with switch 448 switched to its sync. lock position 432, the output of the comparator is used to control VCO 436. The low pass filter 434 besides maintaining proper correction voltage at switch 448 during transfer, also limits the time rate of change so as to produce a reduction in the raster disturbance resulting from establishment of horizontal sync. A known sync. lock system (26a, Fig. 12, not shown) may be utilized in which a high frequency clock and VCO is incorporated into the system.