Chrominance-Locked Clock Generator

14 EE July/August 1988 CHROMINANCE-LOCKED CLOCK GENERATOR OMNI 0311.111110101110* A remarkably simple solution is offered to a problem almost any constructor of a test card or callsign generator, logomat, graphics card, or any other video equipment must have been faced with at some time: phase synchronicity between clock pulses in the system and the chrominance subcarrier. A PLL circuit is described that enables deriving the TV line and field frequency, and a number of other useful signals, from the chrominance frequency, 4.433 MHz. A mystery unravelled! In video equipment, the beneficial ef- fects of phase-locking the central clock oscillator to the chrominance subcarrier are mainly the elimination of annoying digital interference and colour cross- patterning. The improvement can be noticed in the well-known colour bar test chart, in which colour transitions be- come sharply defined rather than blur- red with bands of spurious lines and ran- domly moving coloured spots while long...