DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

In analog interlaced video, 0 V
denotes the start of either field. In
digital video, 0 V for the second field
is unimportant; some people use 0 V
to denote the start of a frame.
Analog SDTV sync,
genlock, and interface 34
In analog SDTV, sync is combined with video and
conveyed by levels “blacker than black.” This chapter
explains the construction of analog sync, and explains
sync separation, which recovers the significant timing
instants associated with an analog video signal, and
genlock, which reconstructs a sampling clock.
In analog video, line sync is achieved by associating,
with every scan line, a line sync (horizontal) datum
denoted 0 H (pronounced zero-H) defined at the
midpoint of the leading (falling) edge of sync. Field and
frame sync is achieved by associating, with every field,
a vertical sync datum denoted 0 V (pronounced zero-V).
Analog sync
Figure 34.1 overleaf illustrates the development of the
combined (vertical and horizontal) sync waveform.
Every line outside the vertical interval starts with
a normal sync pulse having a duration of 4.7 µs. Vertical
sync is identified by a sequence of broad pulses, each
having a duration of half the line time less a full sync
width. The broad pulses are serrated so that a receiver
can maintain horizontal sync during the vertical interval.
When analog sync separators comprised just a few
resistors and capacitors, imperfect vertical sync separation
was prone to exhibit line pairing, where scan lines
from the second field were not laid exactly halfway
between lines of the first field. Line pairs were prone
to be visible. Achieving good interlace required
interposing narrow equalization pulses, each having half
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