DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 24.2 Y’P B P R cube is
formed when R’, G’, and B’
are subject to a particular
3×3 matrix transform. The
valid R’G’B’ unit cube occupies
about one-quarter of the
volume of the Y’P B P R unit
cube. (The volume of the
Y’P BP R unit cube, the outer
boundary of this sketch, is
the same as the volume of
the R’G’B’ cube in Figure 24.1
on page 282; however, the
useful codes occupy only the
central prism here.) Luma
and color difference coding
incurs a penalty in signal-tonoise
ratio, but this disadvantage
is compensated by the
opportunity to subsample.
Izraelevitz, David, and Joshua L.
Koslov, “Code Utilization for
Component-coded Digital Video,”
in Tomorrow’s Television, Proc.
16th Annual SMPTE Television
Conference (White Plains, N.Y.:
SMPTE, 1982), 22–30.
1
4
2
· 220 · 225 2784375
=
3
220 10648000
≈ 0. 261
Yl
G
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It is evident from Figure 24.2 that when R’G’B’ signals
are transformed into the Y’P B P R space of analog video,
the unit R’G’B’ cube occupies only part of the volume
of the unit Y’P BP R cube: Only 1 ⁄4 of the Y’P BP R volume
corresponds to R’G’B’ values all between 0 and 1.
Consequently, Y’P B P R exhibits a loss of signal-to-noise
ratio compared to R’G’B’. However, this disadvantage is
compensated by the opportunity to subsample.
A legal signal is one in which no component exceeds its
reference excursion. Combinations that are R’G’B’-legal
are termed valid. Signals within the Y’P B P R unit cube are
Y’P BP R-legal. However, about 3 ⁄4 of these combinations
correspond to R’G’B’ combinations outside the R’G’B’
unit cube: Although legal, these Y’P BP R combinations
are invalid – that is, they are R’G’B’-illegal.
In digital video, we refer to codewords instead of
combinations. There are about 2.75 million valid codewords
in 8-bit Y’C BC R, compared to 10.6 million in
8-bit studio R’G’B’. If R’G’B’ is transcoded to Y’C BC R ,
then transcoded back to R’G’B’, the resulting R’G’B’
cannot have any more than 2.75 million colors.
CHAPTER 24 LUMA AND COLOR DIFFERENCES 285
Cy
0 Y’ AXIS
1
R
B
Mg
-0.5 0
+0.5
PB AXIS
-0.5
+0.5
P R AXIS
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