DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Subcarrier phase is sometimes inaccurately
called burst phase, because
the adjustment involves rotating the
phase of burst. However, the
primary effect is to adjust the phase
of modulated chroma.
subcarrier be synchronous with 0 H at line 1 of the
frame, within about ±10°. In NTSC, if this requirement is
met at line 1, then the zerocrossing of subcarrier will be
coincident with the analog sync reference point (0 H )
within the stated tolerance on every line. In PAL, the
requirement is stated at line 1, because the phase relationship
changes throughout the frame. Neither NTSC
nor PAL has burst on line 1; SCH must be measureed
from with regenerated subcarrier, or measured from
burst on some other line (such as line 10).
For composite analog switching, it is necessary that the
signals being mixed have matching 0 V ; but in addition,
it is necessary that the signals have matching subcarrier
phase. (If this were not the case, hue would shift during
the transition.) As I have mentioned, cable delay is
accommodated by back-timing. However, with imperfect
cable equalization, cable delay at subcarrier
frequency might be somewhat different than delay at
low frequency. If the source generates zero SCH, you
could match system timing, but have incorrect subcarrier
phase. The solution is to have, at a composite
source, a subcarrier phase adjustment that rotates the
phase of subcarrier through 360°. Equipment is timed
by adjusting system phase to match sync edges (and
thereby, luma position), then adjusting subcarrier
phase to match burst phase (and thereby, the phase of
modulated chroma).
Timing in digital facilities
FIFO: First in, first out. Modern digital video equipment has, at each input,
a buffer that functions as a FIFO. This buffer at each
input accommodates an advance of timing at that input
(with respect to reference video) of up to about
±100 µs. Timing a digital facility involves advancing
each signal source so that signals from all sources arrive
in time at the inputs of the facility’s main switcher. This
timing need not be exact: It suffices to guarantee that
no buffer overruns or underruns. When a routing
switcher switches among SDI streams, a timing error of
several dozen samples is tolerable; downstream equipment
will recover timing within one or two lines after
the instant of switching.
CHAPTER 15 DIGITAL VIDEO INTERFACES 137