Apple Final Cut Pro 6 - Manual (Page 1872)

Appendix A Video Formats 389
V
Video Sample Rate and Bit Depth
The video sample rate of a digital video format determines how often the light
intensity of each video line is sampled.
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.
Sample rate Description
74.25 MHz HD video luma (Y´) sample rate.
37.125 MHz HD video chroma (C
B
C
R
) sample rate. This is half of the luma sample
rate, used for 4:2:2 HD video.
14.3 MHz 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.
13.5 MHz 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.
6.75 MHz This is the sample rate for the color difference channels in 4:2:2
video. This is half of 13.5 MHz.
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.