Apple Final Cut Express 4 - Manual (Page 1071)

Appendix A Video Formats 1071
XIII
Color Sample Ratio
Color sample ratio refers to the ratio of luma (Y´) samples to each color difference
sample (C
B
and C
R
). For example, 4:2:2 color sampling means that for every four pixels
of Y´ data stored, only two C
R
samples and two C
B
samples are stored. By reducing the
number of chroma samples, less color detail is recorded and less bandwidth is required
for storage and transmission. Because we are less sensitive to color detail than we are
to luma detail, subsampling the chroma signal can be considered perceptually lossless.
In absolute terms, chroma subsampling can make processes like chroma keying much
more difficult.
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.
Sampling ratio Description
4:4:4 Each R, G, and B channel, or each Y´, C
B
, and C
R
channel, is sampled
at the same rate. Maximum color detail is maintained.
4:4:4:4 Full sample rate for each color channel, plus a fourth alpha channel
at the full sample rate.
4:2:2 The color channels are subsampled so that the color resolution is
halved. For example, the first pixel in a line contains Y´, C
B
, and C
R
samples. The next pixel contains only a Y´ sample. This pattern
repeats. Most professional video formats use 4:2:2 color subsampling.
4:2:2:4 4:2:2 sample rate for each color channel, plus an alpha channel at
the full sample rate.
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.