GB1427956A - Television waveform generator - Google Patents

Abstract

1427956 Television BRITISH BROADCASTING CORP 21 June 1974 [22 June 1973] 29812/73 Heading H4F Television waveform generator 14 providing a local television waveform in response to a local clock 10 is synchronized to a reference television waveform applied to an input 20 by detecting 31 field synchronizing signals of the reference waveform and by means of a gate 33 determining if such a detected field sync signal is less than N lines early with respect to the local field sync. signal, and by means of a gate 32 determining if it is more than N lines early i.e. late, with respect to the local field sync. signal. If an output is passed by gate 32 the local field period is lengthened by N (e.g. 2 or 4) lines due to counter 34 inhibiting gate 12 from passing clock pulses at twice line frequency to a counter 11 which provides pulses at twice frame i.e. field, rate to generator 14. The local field period is thus successively at field rate lengthened (vide 52 and 53, Fig. 2) by N lines until the external field sync. is early with respect to the local field sync., in which case an output is provided by gate 33 to reset counter 11 thereby causing the local field sync. to jump (e.g. 54, Fig. 2) into synchronism with the external field sync. Local line sync. is synchronized with the reference line sync. by controlling clock 10 in response to the mark-space ratio of the filtered 24 output of a bi-stable 23 triggered by line sync. pulses from generator 14 and cleared by detected 21 external line sync. pulses. When in synchronism the mark-space ratio is arranged to be 1:1. An additional circuit 35 is provided to ensure that the extra N lines due to the lengthening of the locally generated waveform occur during the picture rather than during the production of sync. signals between fields. Both gates 32 and 33 are closed during the first “ line of the locally generated field sync. pulses so that when synchronism is achieved it is not disturbed. In addition to the synchronization operation 52, 53, 54 referred to above, Fig. 2 also shows (a) a direct jump into synchronism at 55 when the external field is an even number of lines late, (b) lines added at 56 and 57 when 2¢ and 1¢ lines late until the external field sync. are less than 2 lines early and then counter 11 reset, in which case two consecutive fields of the same parity are produced as a result of a jump of an odd number of half lines. For a PAL colour television signal, generator 14 is arranged to include a burst controlled oscillator responsive to colour subcarrier signal bursts in the reference waveform to produce a local colour subcarrier signal.