Apple Final Cut Pro 7 - Manual (Page 1301)

Color Difference Channels
The color channels, C
B
and C
R
, are derived by subtracting Y from the R and B signals and
are sometimes referred to generally as B-Y and R-Y, or color difference channels. Analog
video systems such as Betacam SP use P
B
and P
R
channels, and digital video systems use
C
B
and C
R
. Each color channel format performs the same function, but the underlying
specifications are slightly different.
In digital video, the color channels are typically sampled less often than the luma channel,
reducing the video data without noticeable loss of image quality. The ratio of sampling
between the Y
′
C
B
C
R
channels is often written as 4:2:2, 4:1:1, and so on, depending on the
sample rates used. For more information about how color is represented in various video
formats, see
“An Overview of Video Format Characteristics.”
Luma
Luma describes the brightness of video, from absolute black, through the distribution of
gray tones, all the way up to the brightest white. Luma is completely separate from the
color of your video. In fact, if you viewed the luma of a video clip by itself, you would see
a grayscale image completely devoid of color.
Luma is measured by Final Cut Pro as a digital percentage from 0 to 100, where 0
represents absolute black and 100 represents absolute white. Final Cut Pro also allows
you to see super-white levels (levels from 101 to 109 percent) if they exist in your clip.
Although super-white video levels are not considered to be broadcast-safe, many
consumer camcorders record video at these levels anyway.
Black
0% luma 100% 109%
White
Super-white
Note: In analog video, luma is measured in IRE units. These IRE measurements are irrelevant
in Final Cut Pro because it deals only with the digital signal that exists in your computer
as a straight percentage from 0 to 100 for NTSC and PAL video. For more information,
see
“How Analog Video Signals Are Measured.”
1301 Chapter 78 Measuring and Setting Video Levels