RGB, YUV, 4:4:4 and 4:2:2 are terms often used for digital video. While RGB and YCbCr (correct denotation for the digital component signal often called YUV) represent different forms of color signals, 4:4:4 and 4:2:2 are a denotation for chroma sampling.

Color Signals

• RGB means the analogue or digital form of signals for the three color components red, green and blue. RGB signals deliver very high quality images providing more possibilities for postproduction. At the same time RGB signals require high bandwidth and storage space.
• YCbCr is called a component signal and consists of a luma channel (luminance) and two chroma channels (color difference channels), which originate from the RGB signal. Color difference channels are calculated from a weighted difference of blue- or red signal and the luma signal. YCbCr signals require less bandwidth than RGB signals and thus are more common for economical reasons.

Chroma (Sub-)Sampling

The bandwidth required by component signals can be reduced by means of so-called chroma subsampling. The behind this technique utilizes human vision. A reduction of color information has lesser influence on the impression of an image, as long as full luminance information is available. For this reason, the chroma channels of a component signal may be sampled with a lower frequency than the luma channel – i.e. subsampling.

The notation of three or sometimes four digits separated by colons represents the relationship of subsampling.

• The first digit represents the relative horizontal sampling frequency (simply speaking: the amount of information per line) of the luma channel. “4” is just a historical reference to standard definition television and provides a base value.
• The second digit represents the factor for horizontal subsampling (i.e. the relation of the amount of information per line) of the chroma channels.
• The third digit represents the factor for vertical subsampling (i.e. the relation of the amount of information for two adjacent lines) of the chroma channels. In professional video, this digit is identical with the second. Only consumer products (e.g. HDV) make use of this kind of subsampling.
• If a fourth digit is given, it indicates the presence of an alpha channel. An alpha channel contains transparency/opacity information used in postproduction (e.g. keying) and is provided for each pixel without subsampling.

In a 4:4:4 signal, each pixel has full luminance and chrominance information. As the luma component of an RGB signal is a part of all color channels, an RGB signal is always 4:4:4. In a 4:2:2 signal each pixel contains full luminance information, but only every other pixel per line also contains chrominance information. An RGB signal has to be transformed into a color difference signal, such as YCbCr, before it can be subsampled. YCbCr signals can be delivered as a signal using 4:2:2 subsampling, or as a 4:4:4 signal without subsampling.