DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 24.3 shows a time delay
element in the luma path. Luma is
delayed by a time interval equal to
the transit delay of chroma through
the chroma bandlimiting filters.
R
G
Figure 24.3 Conventional
luma/color difference
encoder. Numerical coefficients
here are for SDTV;
different coefficients are
standard for HDTV.
Eq 24.1 Rec. 601 Y’P B P R
encoding matrix (for SDTV)
B
TRANSFER
FUNCTION
⎡ 0. 299 0. 587 0. 114⎤
⎢
⎥
P = ⎢−0.
169 −0.
331 0. 5 ⎥
⎢
⎣
0. 5 −0. 419 −0.
081⎥
⎦
LUMA
WEIGHTED SUM
R’
+0.299 ∑
G’
B’
+0.587
+0.114
ENCODING
MATRIX
COMPENSATING
DELAY
Conventional luma/color difference coding
I explained constant luminance on page 75. True
constant luminance coding remains an intriguing possibility,
but at present all video systems use nonconstant
luminance coding, which I will now describe.
A conventional luma/color difference encoder is shown
in Figure 24.3 above. First, a nonlinear transfer function
is applied to each of the red, green, and blue linear
(tristimulus) components. Then luma is formed as a
weighted sum of gamma-corrected R’, G’, and B’
components. B’-Y’ and R’-Y’ color difference components
are formed by subtraction; in Figure 24.3, scaling
to analog P B and P R components is indicated. Finally,
the color difference components are lowpass filtered.
The gray rectangle in Figure 24.3 groups together the
weighted adder that forms luma with the pair of color
difference subtractors; the combination is equivalent to
matrix multiplication by the 3×3 matrix P shown in
Equation 24.1 in the margin. The numerical values used
in Equation 24.1, in Figure 24.3, and in subsequent
figures in this chapter all reflect the Rec. 601 luma coefficients
used in SDTV. Unfortunately, the coefficients for
HDTV are different; as I will describe in Component
video color coding for HDTV, on page 313.
286 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES
Y’
COLOR
DIFFERENCE
SUBTRACT
+0.564
-0.564
+0.713
-0.713
∑
∑
CHROMA
BANDLIMITING
Y’
P B
P R