DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 28.1 UV components
are scaled so as to
limit the composite (Y’+C)
excursion. UV scaling is
inappropriate if the components
are kept separate.
This sketch reflects the
display of a colorbar test
signal on an NTSC vectorscope;
the colorbar signal
is described on page 535.
If the figure rotates with
respect to burst, a chroma
phase error has occurred;
upon display, this will manifest
itself as a hue error.
The exact values are these:
kB
=
kR
=
1
3
209556997
96146491
221990474
288439473
Incidentally, the ratio k B :k R is
almost exactly 9:16.
-0.5
Yl
BURST
G
R
+0.5
-0.5
UV components
V axis (cos)
+0.5 U axis (sin)
In Component video color coding for SDTV, on page 301,
I outlined the formation of luma and color difference
components. In encoding NTSC, PAL, or S-video, the
color difference components are scaled so that the
eventual composite signal is contained within the
amplitude limits of VHF and UHF television transmitters.
It is standard to limit the composite excursion to
the range [- 1 ⁄3 … + 4 ⁄3] of unity luma excursion. To this
end, the B’–Y’ and R’–Y’ components are scaled
according to Equation 28.1 by factors k B and k R to form
U and V components depicted in Figure 28.1.
U = k ⎛ 601
B− Y ⎞ V = k ⎛ 601
R− Y ⎞
B ' ' ;
⎝ ⎠
R ' '
⎝ ⎠
The scale factors are chosen to satisfy this constraint:
I described B’-Y’, R’-Y’ components for SDTV, on
page 303. The maximum excursions of the individual
color difference components occur at the blue and red
primaries. Since these points are not located on the
axes where scaling takes place, the scale factors are
derived from two simultaneous equations involving B’–
Y’ and R’–Y’. The scale factors were once standardized
to three digits, rounded to 0.493 and 0.877; in the
contemporary SMPTE 170M standard, they are
336 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES
0
Cy
Mg
B
Eq 28.1
1 601 2 2 4
− ≤ Y'± U + V ≤
Eq 28.2
3
3