Apple Final Cut Pro 6 - Manual (Page 1862)

Appendix A Video Formats 379
V
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, video cameras, flat-
panel displays, and video projectors. Red, green, and blue signals can be combined
to make any color, as well as grayscale images ranging 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 proportions similar to
the way human vision perceives those three colors. Therefore, the luma signal
approximates the way the human eye sees brightness in color images. Humans are
most sensitive to the green portion of the visible spectrum, and therefore the luma
channel mostly consists of 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, re-creating
the Y´C
B
C
R
component signal and then the RGB signal for display. Because the
chroma and luma channels are superimposed, 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 serves a similar
function in all formats. Some common names for color difference channels include C
B
, C
R
; R-Y, B-Y; and U,V.