Put simply 4:4:4 and 4:2:2 terms are descriptions of the sample formats used in digital video. In the early 80's tests were done to determine the sample formats and rates for digital video. The eventual sample structure used for SDI video ended up being 4 times the base sample rate chosen.
The first 4 in the 4:2:2 term is for luminance or the black and white information, and this is where most of the picture detail is. Early tests in television human vision discovered a greater sensitivity to black and white information, while the color is filled in with less detailed areas of the human eye. This means you can reduce the color information and your eye cannot really tell. This is what the 2:2 part of 4:2:2 is for. It means the red and blue channels of the video signal are half the bandwidth of the luminance information. Green is not sent, as you can calculate green from red, blue and luminance information.
This color bandwidth reduction has been used for years in broadcast color television, and in fact the color bandwidth of 4:2:2 is much higher than composite video. This all adds up to 4:2:2 being compatible with black and white or composite television, as the color and luminance information is sent separately, while only 2/3 of the data rate is required for about the same visual quality video.
4:4:4 video is similar, but this time all the color information is sent. RGB computer graphics are really 4:4:4. The 4:4:4:4 format adds a key channel.
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