DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Owing to the dependence of the
optimum end-to-end power function
upon viewing conditions,
there here ought to be a user
control for rendering intent –
perhaps even replacing brightness
and contrast – but there isn’t!
480
601
Y’CBCR SDTV
1080
640
720
709
Y’CBCR HDTV
Figure 24.9 Luma/color
difference flavors
1920
The transfer functions of the decoder (or the CRT) are
intertwined with gamma correction. As I explained on
page 81, an end-to-end power function having an
exponent of about 1.25 is appropriate for typical television
viewing in a dim surround. The encoder of
Figure 24.7 imposes a 0.5-power function; the decoder
of Figure 24.8 imposes a 2.5-power. The product of
these implements the end-to-end power function.
When viewing in a light surround, a 1.125 end-to-end
power is appropriate; when driving a CRT, a 0.9-power
function should intervene. When viewing in a dark
surround, a 1.5 end-to-end power is appropriate;
a 1.2-power function should intervene. If the transfer
function of a display device differs from that of a CRT,
then decoding should include a transfer function that is
the composition of a 2.5-power function and the
inverse transfer function of the display device.
If the display primaries match the interchange primaries,
the decoder’s 3×3 tristimulus matrix is not needed.
If a CRT display has primaries not too different from the
interchange primaries, then it may be possible to
compensate the primaries by applying a 3×3 matrix in
the nonlinear domain. But if the primaries are quite
different, it will be necessary to apply the transform
between primaries in the tristimulus domain; see Transforms
among RGB systems, on page 252.
SDTV and HDTV luma chaos
Although the concepts of Y’PBPR and Y’CBCR coding are
identical in SDTV and HDTV, the Rec. 709 standard
has – unfortunately, in my opinion – established a new
set of luma coefficients for HDTV. That set differs
dramatically from the luma coefficients for SDTV specified
in Rec. 601. There are now two flavors of Y’CBCR coding, as suggested by Figure 24.9 in the margin;
I denote the flavors 601Y’CBCR for SDTV, and 709Y’CBCR for HDTV. Similarly, there are two flavors of Y’PBPR for
analog systems, 601Y’PBPR for SDTV, and 709Y’PBPR for
HDTV. (A third luma coefficient set was specified in
SMPTE 240M-1988; though obsolete, that set
continues to be used in legacy 1035i equipment.)
296 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES