4:4:4 vs 4:2:0: Which Chroma Subsampling Do You Need for Your Video Application?
Friday, May 2, 2014
As a distributor specializing in audio / video, we frequently get asked about requirements for video designs. One question we often see is how to determine the right chroma subsampling needed for a video application. Here I will discuss the differences between 4:4:4 and 4:2:0 chroma subsampling, and hopefully help you find the right option for your design.
What is Chroma Subsampling?
Video information is recorded, or sampled, and passed along at a reasonable bandwidth to, in the end, illuminate groups of pixels so together they form a moving image. Video is sampled and transmitted in the form of three components: Y, Cb, and Cr, where Y is the Luma (brightness information), Cb is the blue difference channel (color information), and Cr is the red difference channel (color information). Each type of subsampling is represented by three numbers, indicating how much of the sampled data passes to a small group of pixels consisting of two lines with four pixels each. The first number describes how many pixels on each line get a unique brightness level. The second number describes how many pixels on the upper line get unique color information. The third number describes how many pixels on the lower line get unique color information.
What is the Difference Between 4:4:4 and 4:2:0 Chroma Subsampling?
With 4:4:4 subsampling, each of the four pixels in the upper row gets unique brightness and color information and each of the four pixels in the lower row gets unique brightness and color information. With 4:2:0, each of the four pixels in the upper row gets unique brightness information, but there are only two samples of color information that populate the upper row’s four pixels. Each of the four pixels in the lower row also gets unique brightness, however, there isn’t any new color information for this line; the color information is copied from the upper row. So among eight pixels, there are only two colors passed along, each covering a two by two area, but with each of pixels having their own brightness level.
Why Not 4:4:4 All the Time?
If each of the three components (Y, Cb, and Cr) are sampled equally and transmitted, we are spending two thirds of our available bandwidth passing color information and one third passing brightness information. Video engineers realized the human eye is more sensitive to brightness levels than to color. There is no perceptible difference between 4:4:4 and 4:2:0 when viewed at a normal viewing distance. Thus, video engineers created several standard ways to subsample the color information, allowing them to pass more pixels over the available bandwidth with less color information. A 4:2:0 signal saves fifty percent of the bandwidth compared with the same video at 4:4:4, without affecting the color display.
If you would like advice on which chroma subsampling your application requires, call Symmetry Electronics at (310) 536-6190, or contact us online.