Bit depth – Apple Final Cut Pro 6 User Manual
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The following table shows a list of color sample ratios used in various digital video formats:
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