DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Macrovision is a proprietary,
patented scheme to introduce electrical
disturbances into the sync and
burst elements of composite analog
video so as to defeat subsequent
VHS recording, without causing too
much disruption to receivers and
video monitors. In a sense, it is
a degenerate form of NTSC or PAL.
Degenerate analog NTSC and PAL
In addition to studio-standard NTSC and PAL signals
that I detailed in Chapters 28 and 29, there are three
degenerate forms of composite analog NTSC and PAL
video. The three degenerate modes are classified, along
with studio-standard video, in Table 49.1:
Coherent
subcarrier
Incoherent
subcarrier
Stable timebase Unstable timebase
Studio standard
Industrial,
Low-end consumer
Direct color
(e.g., Type C)
Color-under
(e.g., VHS)
Table 49.1 Composite analog NTSC/PAL degenerate modes
• Video played back from obsolete analog equipment,
such as “direct color” Type-C analog VTRs or consumer
laserdisc players, has an unstable timebase and
coherent subcarrier. Such a signal must be processed
through a timebase corrector (TBC) prior to processing
or even monitoring.
• Upon playback from a VTR that uses the color-under
recording scheme, not only is the timebase unstable,
but color subcarrier is incoherent with sync.
• Consumer devices such as video games, and substandard
industrial equipment, develop signals having
stable timebase but incoherent subcarrier.
Coherent subcarrier
In studio-quality NTSC video, the color subcarrier has
a frequency of exactly 227.5 times the line rate.
Although its phase may be subject to some uncertainty,
its frequency is coherent with line rate, as introduced in
Frequency interleaving, on page 107.
When NTSC subcarrier and sync are generated in
a single circuit, it is simple to arrange for coherence of
the subcarrier and the line rate: The two need simply be
divided from the same master oscillator. In 576i PAL,
subcarrier is considered coherent if there are either
283.75 or 283.7516 subcarrier cycles per line. Coherence
is somewhat difficult to achieve in PAL owing to
the +25 Hz offset.
CHAPTER 49 CONSUMER ANALOG NTSC AND PAL 581