DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

In a vacuum, light travels
0.299972458 m – very nearly
one foot – each nanosecond.
3 579545 5 63
. = ⋅
88
System phase advances or delays all
components of the signal. Historically,
horizontal phase (or
h phase) altered sync and luma but
left burst and subcarrier untouched.
Timing in analog facilities
Signals propagate through coaxial cable at a speed
between about 0.66 and 0.75 of the speed of light in
a vacuum. Time delay is introduced by long cable runs,
and by processing delay through equipment. Even over
a long run of 300 m (1000 ft) of cable, only a microsecond
or two of delay is introduced – well under 1 ⁄4 of
a line time for typical video standards. (To reach a delay
of one line time in 480i or 576i would take a run of
about 12 km!) To compensate typical cable delay
requires an adjustment of horizontal timing, by just
a small fraction of a line time.
In analog video, these delays are accommodated by
advancing the timing at each source, so that each signal
is properly timed upon reaching the production
switcher. In a medium-size or large facility, a single sync
generator (or a pair of redundant sync generators)
provides house sync, to which virtually everything else
in the facility is locked with appropriate time advance
or delay. To enable a seamless switch from a network
source to a local source in the early days of television
networks, every television station was locked to timing
established by its network! Each network had an atomic
clock, generating 5 MHz. This was divided to subcarrier
using the relationship in the margin.
Many studio sources – such as cameras and VTRs – can
be driven from a reference input that sets the timing of
the primary output. This process was historically
referred to as “sync generator locking,” or nowadays, as
genlock. In the absence of a reference signal, equipment
is designed to free-run: Its frequency will be
within tolerance, but its phase will be unlocked.
In studio equipment capable of genlock, with factory
settings the output signal emerges nominally synchronous
with the reference. Studio equipment is capable of
advancing or delaying its primary output signal with
respect to the reference, by perhaps ± 1 ⁄4 of a line time,
through an adjustment called system phase. Nowadays,
some studio video equipment has vertical processing
that incorporates line delays; such equipment introduces
delay of a line time, or perhaps a few line times.
CHAPTER 15 DIGITAL VIDEO INTERFACES 135