Your questions: What is chroma subsampling?

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Our community members have asked us about chroma subsampling. What is it? Why is it done? What do the numbers mean? Here are all the answers…

What is chroma subsampling?

Suppose you’ve got a digital signal with a lot of data, carrying lots of information. If there were a way to reduce the size of that signal, you would reduce transmission time and file size.

Chroma subsampling is one way to compress a digital signal and it works by manipulating colour information. By encoding less colour information than brightness information, TV signals use less bandwidth. The human eye is actually more sensitive to brightness than to colour, so the difference in image quality is barely noticeable.

How does it work?

Digital images are made up of pixels (squares). With chroma subsampling, each pixel has its own luma (brightness) information, but its chroma (colour) information is shared between a pixel group. The more chroma subsampling, the more pixels in a group sharing colour data.

If you’ve ever seen the notation Y’CbCr, this refers to the format in which consumer video encodes the RGB colour information used by TV screens. Y stands for brightness; Cb and Cr refer to colour difference.

 

What do the numbers mean?

4:4:4 means there has been no subsampling. This is the case in high-end HD cameras.

4:2:2 uses subsampling to produce a good quality but less detailed image. It is used in editing codecs and Sony cameras that film XDCAM 4:2:2.

Most smartphones, compact cameras, DV and most MPEGs use 4:2:0, which means more subsampling has been employed.

The numbers tell you how much subsampling is happening. Think of an image as having two rows, 4 pixels across. The first number tells you your image is a grid 4 pixels wide. The second explains how many pixels share colour information in the top row, and the third tells you how many share in the bottom row. You’ll see that in 4:2:0, the bottom row has no colour information at all.

How 4K avoids chroma subsampling problems

Dropping more colour information can have disadvantages, like visible pixels or jagged lines. However, when chroma subsampling is done on 4K – a signal that contains more information – the result is that less data is lost. It was previously only possible to achieve true 1080p video with professional cameras, but for the first time, you can both shoot extremely high quality and get a high-quality compressed image as well.