HD

Appendix A Video Formats 1029
XIII
The luma (black-and-white channel) and chroma (color channels) information can be
recorded and transmitted several different ways in a video signal.
 RGB (Red, Green, Blue): This is the native format for most computer graphics and
video files. This signal is also used inside traditional color CRTs and flat-panel displays.
Red, green, and blue signals can be combined to make any color, as well as grayscale
images from black (no signal on any channel) to white (full signal on every channel).
RGB signals do not have a separate luma channel, because black-and-white signals
can be represented by equal amounts of R, G, and B signals.
 Component YUV or Y´C
B
C
R
: This three-channel signal has a luma (Y’) signal and two
color difference channels (C
B
and C
R
)
1
. Component video was invented in the 1950s as
a way of making color television signals compatible with existing black-and-white
televisions. Black-and-white televisions could use the luma signal while color
televisions could convert Y’, C
B
, and C
R
back to RGB for display.
The luma signal is derived by combining R, G, and B signals in similar proportions to
the way human vision is sensitive to those three colors. Therefore, the luma signal
approximates the same detail we see for various colors in the spectrum, which
happens to be most sensitive to detail in the green channel. The color difference
channels are so named because they are derived from RGB by subtracting signals
from the luma channel for each of the color channels (for example, R-Y or B-Y).
 S-Video (Y/C): An S-Video signal is also considered a component video signal
because the luma and chroma signals are separate. However, the C signal is derived
by combining the C
B
and C
R
component signals, which reduces the quality of the
color channel compared to Y´C
B
C
R
.
 Composite: The luma (Y´) and chroma (C) signals are combined into a single,
composite video signal for broadcast. The chroma signal is placed on a color
subcarrier frequency related to the main luma frequency. This method of
superimposing color information on top of the black-and-white information indicates
that this format originated in the early days of color television, when black-and-white
TV compatibility was critical for widespread adoption.
Black-and-white televisions are unaware of the color subcarrier, and so only the luma
(Y´) channel is shown. Color televisions reverse the composite process, recreating the
Y´C
B
C
R
component signal and then the RGB signal for display. Because the chroma
and luma channels are combined, they do not separate perfectly, causing artifacts in
the resulting image.
1.
The pair of color difference channels has different names depending on the particular format, but they all serve a
similar function. Some common names for color difference channels include: C
B
, C
R
; R-Y, B-Y; or U,V.