DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Luma and color differences can be
computed from R’, G’, and B’
through a 3×3 matrix multiplication.
Y’P BP R
Y’C BC R
Color difference coding (chroma)
In component video, three components necessary to
convey color information are transmitted separately.
Rather than conveying R’G’B’ directly, the relatively
poor color acuity of vision is exploited to reduce data
capacity accorded to the color information, while maintaining
full luma detail. First, luma is formed according
to Marginal note (or for HDTV, Marginal note ). Then,
two color difference signals based upon gammacorrected
B’ minus luma and R’ minus luma, B’-Y’ and
R’-Y’, are formed by “matrixing.” Finally, subsampling
(filtering) reduces detail in the color difference (or
chroma) components, as I will outline on page 93.
Subsampling incurs no loss in sharpness at any reasonable
viewing distance.
In component analog video, B’-Y’ and R’-Y’ are scaled
to form color difference signals denoted P B and P R ,
which are then analog lowpass filtered (horizontally) to
about half the luma bandwidth.
In component digital video, M-JPEG, and MPEG, B’-Y’
and R’-Y’ are scaled to form C B and C R components,
which can then be subsampled by a scheme such as
4:2:2 or 4:2:0, which I will describe in a moment.
Y’UV In composite NTSC or PAL video, B’-Y’ and R’-Y’ are
scaled to form U and V components. Subsequently, U
and V are lowpass filtered, then combined into
a modulated chroma component, C. Luma is then
summed with modulated chroma to produce the
composite NTSC or PAL signal. Scaling of U and V is
arranged so that the excursion of the composite signal
(Y’+C) is constrained to the range - 1 ⁄3 to + 4 ⁄3 of the
unity excursion of luma. U and V components have no
place in component analog or component digital video.
Y’IQ Composite NTSC video was standardized in 1953 based
upon I and Q components that were essentially U and V
components rotated 33° and axis-exchanged. It was
intended that excess detail would be removed from the
Q component so as to improve color quality. The
scheme never achieved significant deployment in
receivers, and I and Q components are now obsolete.
CHAPTER 10 INTRODUCTION TO LUMA AND CHROMA 89