Apple Final Cut Pro 7 - Manual (Page 1896)

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.
1896 Appendix B Video Formats