DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

R
G
B
0.5
R’
G’
B’
Figure 8.11 Subsampled
chroma components
Figure 8.12 Failure to adhere
to constant luminance is
evident in the dark band in
the green-magenta transition
of the colorbar test signal.
The notation 4:2:2 has come to
denote not just chroma subsampling,
but a whole set of SDTV
interface parameters.
Y’
[P] [P -1
]
CB
CR
In Figure 8.11 above, I show the practical block diagram
of Figure 8.10, augmented with subsampling filters in
the chroma paths. With nonconstant luminance coding,
some of the relative luminance traverses the chroma
pathways. Subsampling not only removes detail from
the color components, it also removes detail from the
“leaked” relative luminance. Consequently, relative
luminance is incorrectly reproduced: In areas where
luminance detail is present in saturated colors, relative
luminance is reproduced too dark, and saturation is
reduced. This is the penalty that must be paid for lack
of strict adherence to the Principle of Constant Luminance.
These errors are perceptible by experts, but they
are very rarely noticeable – let alone objectionable – in
normal scenes. The departure from theory is apparent in
the dark band appearing between the green and
magenta color bars of the standard video test pattern,
depicted in Figure 8.12 in the margin.
To summarize signal encoding in video systems: First,
a nonlinear transfer function, gamma correction, comparable
to a square root, is applied to each of the linear R,
G, and B tristimulus values to form R’, G’, and B’. Then,
a suitably weighted sum of the nonlinear components is
computed to form the luma signal (Y’). Luma approximates
the lightness response of vision. Color difference
components blue minus luma (B’-Y’) and red minus
luma (R’-Y’) are formed. (Luma, B’-Y’, and R’-Y’ can be
computed from R’, G’, and B’ simultaneously, through
a 3×3 matrix.) The color difference components are
then subsampled (filtered), using one of several
schemes – including 4:2:2, 4:1:1, and 4:2:0 – to be
described starting on page 87.
80 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES
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