Bit Depth
The number of bits used per sample determines how accurately the sample is stored,
as well as 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. Two 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 usually have 24-bit
color plus 8 more bits for an alpha channel.
Note: Still images using 16 bits per color channel, or 48 bits per RGB pixel, are becoming
more common. However, most video formats use 8 or 10 bits per color channel.
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.
4:1:1
The color is subsampled so that the color resolution is quartered.
The first pixel in a line contains Y´, C
B
, and C
R
samples. The next
three pixels only contain Y´ samples. This pattern repeats.
4:2:0
This ratio indicates that the C
B
and C
R
channels are subsampled both
horizontally (as in 4:2:2) and vertically. This reduces color resolution
in both the horizontal and vertical dimensions compared to 4:2:2,
which only reduces horizontal chroma resolution.
There are several methods for locating C
B
and C
R
samples relative
to Y´ samples, yielding several different 4:2:0 formats.
Sampling ratio
Description
Sample ratio
Video formats
4:4:4
HDCAM SR
Most RGB computer graphics files (implicit)
4:2:2
Digital Betacam, D-1, D-5, DVCPRO HD, DVCPRO 50, and HDCAM SR
3:1:1
HDCAM
4:1:1
NTSC DV, NTSC DVCAM, and DVCPRO
4:2:0
PAL DV, PAL DVCAM, DVD, and HDV
Appendix A
Video Formats
391
V
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 Volume III, Chapter 29, “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 Volume III, Chapter 29, “Rendering and Video
Processing Settings.”