The video signal consists of the luminance signal for brightness and the chrominance signal for color. Since the resolving power of the human eye is much lower for color than for brightness information, the chrominance signals are transmitted with a much lower bandwidth than the luminance signal "Y" for all video-relevant color models.
With a video bandwidth of 5 MHz for the luminance signal, only 1 MHz to 2 MHz is available for the color difference signals, "U" or Cb and "V" or Cr. This also means that the color difference signals can be quantized with a lower sample depth without any visible loss of quality, resulting in a corresponding reduction in data.
Subsampling with the luminance and two chrominance signals
The color subsampling or color subsampling tells in which ratio the luminance sampling stands to the color sampling. The specification 4:x:x means that the luminance is quantized with a factor of 4, Cb with the sampling ratio of x, and Cr also with the sampling ratio of x. The base sampling rate is 3.375 MHz, which means that the quadruple sampling rate is 13.5 MHz. Luminance is sampled at this sampling rate.
With 4:4:4 subsampling, the resolution of the two color difference signals is identical to that of the luminance signal. Each pixel is represented with three values and the three values are transmitted with the same frequency. The three color values of the RGB signal or of YcbCr are thus sampled and transmitted with equal frequency. All signals are sampled at 13.5 MHz. Although this subsampling provides the best image quality, it is rarely used, and if so, then in production to keep color losses small.
With 4:2:2 color subsampling, the luminance value is sampled at each pixel, and the two crominance signals are sampled at every other pixel. The luminance signal is sampled at 13.5 MHz and the color difference signals "U" and "V" at 6.75 MHz. This color subsampling complies with ITU-R BT.601
With a subsampling of 4:1:1, the color difference signals are sampled every fourth sample of the luminance signal. This means that four color pixels have a U and V value. The sampling rate for the two chrominance signals is thus only a quarter of that of the luminance signal.
In the case of 4:2:0 subsampling, the chrominance signals are sampled for four adjacent pixels arranged in a square. As with the other subsampling methods, the luminance signal is sampled at each pixel. In principle, this is a combination of two methods applied to the even and odd lines: The odd lines are sampled at a ratio of 4:2:2, and the even lines are sampled at a ratio of 4:0:0. As with 4:1:1 sampling, the data reduction is 50%. 4:2:0 subsampling is used with most DV formats in the PAL television standard.