Apple Final Cut Express HD - Manual (Page 1039)

Appendix A Video Formats 1039
XIII
Bit Depth
The number of bits used per sample determines how accurately the sample is stored,
and how much intensity variation is possible within the signal. For example, a video
signal with a bit depth of only 1 bit can have either a value of 0 or 1, resulting in only
black or white pixels. 2 bits per sample results in four possible values: 00, 01, 10, or 11,
or any of four shades of gray (or some other color) per sample.
Most digital video formats use a minimum of 8 bits per color channel, or 256
gradations of intensity. RGB images are traditionally described by the total bits used per
pixel (8 bits per channel x 3 channels = 24 bits). 32-bit RGB images refer to 24-bit color
plus 8 more bits for an alpha channel. Images with higher bit depth are becoming
more common, using 16 bits per color channel, or 48 bits per RGB pixel.
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
R
,
and 8 bits for C
B
). 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. Some formats, such as Digital Betacam, record internally at
8-bit but can accept and output 10-bit video.
Internally, Final Cut Express HD 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 combining
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 Express HD, 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 ” on page 877.