Apple Final Cut Pro 7 - Manual (Page 1907)

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
“Rendering and Video Processing Settings.”
Internally, Final Cut Pro 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. In Final Cut Pro, the Video Processing tab in the
Sequence Settings window allows you to choose the rendering bit depth for a sequence.
For more information, see
“Rendering and Video Processing Settings.”
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
“Rendering
and Video Processing Settings.”
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.
1907 Appendix B Video Formats