Apple Final Cut Express 4 - Manual (Page 1072)

1072 Part XIII Appendixes
Video signal bit depth is usually described per channel. For example, DV and
DVCPRO HD use 8 bits per color component (in other words, 8 bits for Y
´ , 8 bits for C
B
,
and 8 bits for C
R
). Other formats, such as D-5, use 10 bits per component. This provides
1024 possible gradations instead of 256, which means much more subtle variations in
intensity can be recorded.
In fact, 8-bit Y´C
B
C
R
video does not use all 256 codes to represent picture information.
Black is stored as code 16 and white is code 235. Codes 1–15 and 236–254 are retained
for footroom and headroom, respectively. These areas allow for quick spikes in the signal
caused by filtering in analog-to-digital conversions and, in the case of white levels, can
prevent clipping for highlights that may exceed 235 (white). Levels above 235 are
sometimes referred to as super-white levels. For more information about super-white
levels, see Chapter 57, “
Rendering and Video Processing Settings ,” on page 899.
Internally, Final Cut Express can do pixel calculations using 32-bit floating-point
precision, which allows for very accurate calculations without rounding errors. This
leads to much more accurate color reproduction when applying filters and
compositing layers of video. This is especially important when you are going to show
your movie on film or broadcast-quality video monitors.
Perceptual Coding and Gamma
The limited number of brightness steps in 8-bit digital video requires efficient use
of the 256 available codes. Because perception of brightness follows a power law
function, humans are more sensitive to absolute intensity changes in dark areas. In
other words, the amount of light required to make a perceptual shift in brightness
increases exponentially. Therefore, a gamma correction is applied to video so that the
step between each code represents a perceptual shift in brightness. Without this
gamma correction, the darker areas would appear to abruptly jump from one
brightness level to the next (“banding”) and white levels would waste many codes with
imperceptible brightness shifts. This gamma correction is reversed by video monitors
so that the viewer sees the original light intensity of the image. For more information
about gamma, see Chapter 57, “
Rendering and Video Processing Settings ,” on page 899.
Video Compression
Once a video signal is digital, it requires a large amount of storage space and
transmission bandwidth. To reduce the amount of data, several strategies are employed
that compress the information without negatively affecting the quality of the image.
Some methods are lossless, meaning that no data is lost, but most are lossy, meaning
that information is thrown away that can’t be retrieved.