Apple Final Cut Pro 6 - Manual (Page 1294)

Chapter 26 Measuring and Setting Video Levels 509
III
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
, and 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 Volume IV, Appendix A, “Video Formats.”
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.
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 ” on page 534.
Black
0% luma 100% 109%
White
Super-white