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DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

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A Naive combined sync establishes horizontal timing using a<br />

relatively narrow pulse each line time. Here, the duration of<br />

the line sync pulse is about 7.5% of the line time. Vertical sync<br />

is signaled by a wide pulse. The vertical pulse here, having<br />

a duration of one line time, has poor noise immunity.<br />

B Noise immunity is improved by stretching the vertical pulse<br />

to three lines. However, line sync is now absent during vertical<br />

sync: Horizontal scanning could become unlocked during this<br />

time. If accurate line sync wasn’t reestablished by the first<br />

picture line, the top of the picture would be disturbed.<br />

C Line sync during vertical blanking is maintained by serrating<br />

the vertical pulse, and by appending a narrow pulse at its end.<br />

Every line now commences with a negative-going sync edge.<br />

The vertical pulses are broad pulses; the start of each broad<br />

pulse is aligned with the associated line sync datum, 0 H .<br />

D Interlace is achieved by offsetting the first broad pulse of the<br />

second field to start halfway between line syncs. If simple<br />

analog circuits were used with this waveform, proximity of the<br />

neighboring line syncs would disturb recovery of the first<br />

negative-going vertical pulse.<br />

E Twice line-rate pulses are introduced before, during, and<br />

after vertical sync, so that sync information in within a few<br />

lines of the vertical pulses has an identical pattern in both<br />

fields. The lines of the second field are thereby accurately<br />

placed between lines in the first field, preventing line pairing.<br />

F Fully developed sync for the second field of 480i29.97 video<br />

is shown here; sync for 576i25 differs in several details<br />

described in the text. This is the sync waveform – some<br />

people call it composite sync, but that’s confusing because<br />

sync is independent of the composite color-coding scheme.<br />

G<br />

400 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES

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