DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

The mismatch between the
primaries and the luma coefficients
of SDTV has little practical
significance; however, the
mismatch of luma coefficients
between SDTV and HDTV has
great practical significance!
primary sets are slightly different; both sets very nearly
match the primaries of Rec. 709.) Despite the change in
primaries, the luma coefficients for 480i and 576i
video have remained unchanged from the values that
were established in 1953. As a consequence of the
change in primaries, the luma coefficients in SDTV no
longer theoretically match the primaries. The mismatch
has little practical significance.
Rec. 709 luma
International agreement on Rec. 709 was achieved in
1990 on the basis of “theoretically correct” luma coefficients
derived from the Rec. 709 primaries:
709
Y'= 0. 2126 R'+ 0. 7152 G'+ 0. 0722 B'
Eq 24.7
SMPTE 240M-1988 luma
Two years before Rec. 709 was adopted, SMPTE standardized
luma coefficients for 1035i30 HDTV that were
“theoretically correct” for the SMPTE RP 145 primaries
in use at the time:
240
Y'= 0. 212 R'+ 0. 701G'+ 0. 087 B'
Eq 24.8
Following the establishment of Rec. 709, the successors
to SMPTE 240M – such as SMPTE 274M for
1920×1080 HDTV – specified the primaries, transfer
function, and luma coefficients of Rec. 709. However,
provisions were made in these standards to accommodate
the 240M parameters as an “interim implementation,”
and the 240M parameters remain in use in some
of the HDTV equipment that is deployed as I write this.
The most recent revision of SMPTE 274M dispenses
with the “interim implementation,” and embraces
Rec. 709.
Chroma subsampling, revisited
The purpose of color difference coding is to enable
subsampling. In analog video, the color difference
components are subject to bandwidth reduction
through the use of analog lowpass filters; horizontal
color detail is removed. In digital video, the chroma
components are subsampled, or decimated, by filtering
followed by the discarding of samples. Figure 10.3,
Chroma subsampling, on page 90, sketches several
292 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES