DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 29.7 Dot crawl is exhibited
when saturated colors sideby-side
are subject to NTSC
decoding. Dot crawl is most
evident on the green-magenta
transition of the colorbar signal.
Figure 29.8 Hanging dots are
evident when the colorbar
signal has been subject to NTSC
decoding by a simple two-line
nonadaptive comb filter.
image, comb filtering works well. However, to the
extent that either luma or chroma changes vertically,
chroma leaks (“crosses”) into luma, and luma leaks
(“crosses”) into chroma. A decoder with a notch filter
will produce cross-color artifacts that may appear as
swirling rainbows, when luma occupying frequencies in
the range of subcarrier “crosses into” – and is mistakenly
decoded as – chroma. A notch filter can also introduce
cross-luma artifacts, when chroma “crosses into” –
and is mistakenly decoded as – luma.
Consider a set of image rows, each containing the same
large, abrupt change in color – say from one colored
vertical bar to another in the colorbar test pattern to be
explained in Colorbars, on page 535. Most decoders will
mistakenly interpret some of the power in this transition
to be luma. The mistakenly decoded luma will not
only invert in phase line to line, it will also invert frame
to frame. The frame-rate inversion, combined with
interlace, produces a fine pattern of dots, depicted in
Figure 29.7. The dots apparently travel upward along
the transition at a rate of one image row per field time.
(In 480i, each dot takes about eight seconds to traverse
the height of the image.) This particular cross-luma artifact
is called dot crawl. It can be avoided in an NTSC
decoder by the use of a comb filter.
Another cross-luma artifact is apparent when the
SMPTE colorbar test pattern is decoded using a simple
2-line (1H) comb filter. About 2 ⁄3 of the way down the
pattern, the image contains highly saturated complementary
colors that abut vertically: There is an abrupt
change from a line containing a set of saturated colors
to a line containing the same colors in a different order.
When decoded by a simple 2-line NTSC comb filter,
each abrupt vertical transition contains power that
decodes to a strong luma component at the subcarrier
frequency. On a monitor with sufficiently high resolution,
stationary, horizontal patterns of hanging dots,
depicted schematically in Figure 29.8, are displayed at
several of the transitions. The artifact is strikingly
obvious when colorbars are displayed on a studio
monitor equipped with a comb filter.
354 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES