We often see words like 4:4:4, 4:2:2, 4:2:0 in the codec information. For example, controller A supports 4K@30Hz 4:4:4, and controller B supports 4K. @30Hz 4: 2: 2. No matter 4:4:4, 4:2:2, 4:2:0, they all describe the way of image compression and sampling, so what is the difference between them? On this page, we will introduce you to the concepts related to chroma sampling.
What is Chroma Subsampling?
Chroma subsampling is a type of compression that reduces the color information in a signal in favor of luminance data. This reduces bandwidth without significantly affecting picture quality.
A video signal is split into two different aspects: luminance information and color information. Luminance, or luma for short, defines most of the picture since contrast is what forms the shapes that you see on the screen. For example, a black and white image will not look less detailed than a color picture. Color information, chrominance, or simply chroma is important as well, but has less visual impact. What chroma subsampling does is reduce the amount of color information in the signal to allow more luminance data instead. This allows you to maintain picture clarity while effectively reducing the file size up to 50%. In the YUV format, luma is only 1/3rd of the signal, so reducing the amount of chroma data helps a lot. Because of bandwidth limitations from internet speeds and HDMI, this makes for much more efficient use of current systems.
4: 4: 4 VS 4: 2: 2 VS 4: 2: 0
The first number (4 in this example) refers to the size of the sample. The following two numbers refer to chroma. They all define horizontal and vertical sampling respectively relative to the first number.
The signal with a chroma of 4:4:4 is not compressed (so no sub-sampling is performed), and the brightness and color data are completely transmitted. In the four-by-two pixel array, the chromaticity of 4:2:2 is half of 4:4:4, and the chromaticity information of 4:2:0 is a quarter of the available colors. The horizontal sampling rate of 4:2:2 signals is half, but the vertical sampling rate is still complete. On the other hand, 4:2:0 will only sample half of the colors of the pixels in the first row, and completely ignore the second row of samples.
