Color Sample Ratio
Color sample ratio refers to the ratio of luma (Y´) samples to each color difference
). For example, 4:2:2 color sampling means that for every four pixels
of Y´ data stored, only two C
samples and two C
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
HD video luma (Y´) sample rate.
HD video chroma (C
) sample rate. This is half of the luma sample
rate, used for 4:2:2 HD video.
Early NTSC digital video recorders sampled video at exactly four
times the frequency of the color subcarrier signal (3.58 MHz x 4).
This is the origin of the 4 in color sample ratios such as 4:2:2.
This is the sample rate for the luma (Y´) channel for SD digital video.
This sample rate was chosen to work with both NTSC and PAL digital
video. The 4 in 4:2:2 is now represented by this sample rate.
This is the sample rate for the color difference channels in 4:2:2
video. This is half of 13.5 MHz.
Each R, G, and B channel, or each Y´, C
, and C
channel, is sampled
at the same rate. Maximum color detail is maintained.
Full sample rate for each color channel, plus a fourth alpha channel
at the full sample rate.
The color channels are subsampled so that the color resolution is
halved. For example, the first pixel in a line contains Y´, C
, and C
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 sample rate for each color channel, plus an alpha channel at
the full sample rate.
The following table shows a list of color sample ratios used in various digital video formats: