DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 45.2 Colorbar R’G’B’
primary components in
100/0/
75/
SMPTE colorbars have amplitude
of 75 IRE, denoted
75/0/75/0. A variation
denoted 100/0/75/0, whose
R’, G’, and B’ waveforms are
R’
0
sketched here, places the
white bar at 100 IRE. Other
variations have different
amplitudes for the uncolored
and colored bars. The nota-
G’
tion is described on page 539.
B’
Figure 28.1, on page 336, sketches
a vectorscope display of the U and V
chroma components of colorbars in
NTSC; a comparable diagram for
PAL is shown in Figure 28.8, on
page 344. Vectorscope diagrams of
chroma in component systems are
shown in Component video color
coding for SDTV, on page 301, and
Component video color coding for
HDTV, on page 313.
Figure 45.2 above shows the R’G’B’ components that
produce the upper 2 ⁄3 of the frame of SMPTE colorbars.
Each scan line is based upon a binary sequence of
red, green, and blue values either zero or unity. The
components are arranged in order of their contributions
to luma, so that the eventual luma component
decreases from left to right. (The narrow, central region
of SMPTE bars has the count sequence reversed, and
the green component is forced to zero.)
In studio equipment, in component video, and in PAL
broadcast, the processing, recording, and transmission
channels can accommodate all encoded signals that can
be produced from mixtures of R’G’B’ where each
component is in the range 0 to 1. The 100% colorbar
signal exercises eight points at these limits.
Fully saturated yellow and fully saturated cyan cause
a composite PAL signal to reach a peak value 133 1 ⁄3%
of reference white. However, as detailed on page 337,
an NTSC transmitter’s composite signal amplitude is
limited to 120% of reference white. If 100% bars were
presented to an NTSC transmitter, clipping would
result. To avoid clipping, 75% bars are ordinarily used
to test NTSC transmission. The white bar comprises
primaries at 100%, but the other bars have their
primary components reduced to 75% so as to limit their
composite NTSC peak to the level of reference white.
Figure 45.3 shows the NTSC composite waveform of
a scan line in the upper region of 75% colorbar.
536 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES