DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

For details concerning SCH,
see page 512.
To compensate line delays, system phase must now
accommodate adjustment of vertical delay as well as
horizontal. The adjustment is performed by matching
the timing of the 50%-points of sync. Misadjustment of
system phase is reflected as position error.
A studio sync generator can, itself, be genlocked.
A large facility typically has several sync generators
physically distributed throughout the facility. Each
provides local sync, and each is timed according to its
propagation delay back to the central switching or
routing point.
If a piece of studio equipment originates a video signal,
it is likely to have adjustable system phase. However, if
it processes a signal, and has no framestore, then it is
likely to exhibit fixed delay: It is likely to have no
genlock capability, and no capability to adjust system
phase. Delay of such a device can be compensated by
appropriate timing of its source. For example, a typical
video switcher has fixed delay, and no system phase
adjustment. (It has a reference input whose sync
elements are inserted onto the primary outputs of the
switcher, but there is no genlock function.)
A routing switcher is a large matrix of crosspoint
switches. A routing switcher is designed so that any
path through the switcher incurs the same fixed delay.
Timing in composite analog NTSC and PAL
NTSC modulation and demodulation work properly
provided that burst phase and modulated subcarrier
phase remain locked: Color coding is independent of
the phase relationship between subcarrier and sync.
If two signals are to be switched or mixed, though, their
modulated subcarrier phase (and therefore their burst
phases) must match – otherwise, hue would shift as
mixing took place. But the phase of luma (and therefore
of the analog sync waveform) must match as well –
otherwise, picture position would shift as mixing took
place. These two requirements led to standardization of
the relationship of subcarrier to horizontal (SCH) phase.
It is standard that the zerocrossing of unmodulated
136 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES