4:1:1

HD

This terminology has come into common usage to describe both high definition and standard definition systems where the ratio between luminance and chrominance samples is 4 to 1 (1 set of color difference samples (R-Y,B-Y) for every 4 luminance samples (Y)). In its original application in standard definition video signals, it represents a system where the luminance (Y) is sampled at 13.5 MHz and the R-Y,B-Y channels are each sampled at 3.75 MHz.